diff --git a/rtengine/improccoordinator.cc b/rtengine/improccoordinator.cc
index 37a81f3d1..4927c4e1f 100644
--- a/rtengine/improccoordinator.cc
+++ b/rtengine/improccoordinator.cc
@@ -1640,31 +1640,13 @@ void ImProcCoordinator::updatePreviewImage(int todo, bool panningRelatedChange)
         }
 
         if (hListener) {
-            updateLRGBHistograms();
-            updateWaveforms();
-            hListener->histogramChanged(
-                histRed,
-                histGreen,
-                histBlue,
-                histLuma,
-                histToneCurve,
-                histLCurve,
-                histCCurve,
-                /*histCLurve,
-                 * histLLCurve,*/
-                histLCAM,
-                histCCAM,
-                histRedRaw,
-                histGreenRaw,
-                histBlueRaw,
-                histChroma,
-                histLRETI,
-                waveformScale,
-                waveformWidth,
-                waveformRed.get(),
-                waveformGreen.get(),
-                waveformBlue.get()
-            );
+            if (hListener->updateHistogram()) {
+                updateLRGBHistograms();
+            }
+            if (hListener->updateWaveform()) {
+                updateWaveforms();
+            }
+            notifyHistogramChanged();
         }
     }
 
@@ -1765,6 +1747,33 @@ void ImProcCoordinator::setScale(int prevscale)
 }
 
 
+void ImProcCoordinator::notifyHistogramChanged()
+{
+    if (hListener) {
+        hListener->histogramChanged(
+            histRed,
+            histGreen,
+            histBlue,
+            histLuma,
+            histToneCurve,
+            histLCurve,
+            histCCurve,
+            histLCAM,
+            histCCAM,
+            histRedRaw,
+            histGreenRaw,
+            histBlueRaw,
+            histChroma,
+            histLRETI,
+            waveformScale,
+            waveformWidth,
+            waveformRed.get(),
+            waveformGreen.get(),
+            waveformBlue.get()
+        );
+    }
+}
+
 void ImProcCoordinator::updateLRGBHistograms()
 {
 
@@ -2305,4 +2314,20 @@ void ImProcCoordinator::setHighQualComputed()
     highQualityComputed = true;
 }
 
+void ImProcCoordinator::updateWaveform()
+{
+    if (hListener) {
+        updateWaveforms();
+        notifyHistogramChanged();
+    }
+}
+
+void ImProcCoordinator::updateHistogram()
+{
+    if (hListener) {
+        updateLRGBHistograms();
+        notifyHistogramChanged();
+    }
+}
+
 }
diff --git a/rtengine/improccoordinator.h b/rtengine/improccoordinator.h
index 367f12a43..92b5e9346 100644
--- a/rtengine/improccoordinator.h
+++ b/rtengine/improccoordinator.h
@@ -54,7 +54,7 @@ class Crop;
   * but using this class' LUT and other precomputed parameters. The main preview area is displaying a non framed Crop object,
   * while detail windows are framed Crop objects.
   */
-class ImProcCoordinator final : public StagedImageProcessor
+class ImProcCoordinator final : public StagedImageProcessor, public HistogramObservable
 {
 
     friend class Crop;
@@ -199,6 +199,7 @@ protected:
 
     MyMutex minit;  // to gain mutually exclusive access to ... to what exactly?
 
+    void notifyHistogramChanged();
     void reallocAll();
     void updateLRGBHistograms();
     void updateWaveforms();
@@ -454,7 +455,11 @@ public:
     }
     void setHistogramListener (HistogramListener *h) override
     {
+        if (hListener) {
+            hListener->setObservable(nullptr);
+        }
         hListener = h;
+        h->setObservable(this);
     }
     void setAutoCamListener  (AutoCamListener* acl) override
     {
@@ -555,6 +560,8 @@ public:
 
     } denoiseInfoStore;
 
+    void updateHistogram() override;
+    void updateWaveform() override;
 };
 
 }
diff --git a/rtengine/rtengine.h b/rtengine/rtengine.h
index 30fa2ec67..4121b4f49 100644
--- a/rtengine/rtengine.h
+++ b/rtengine/rtengine.h
@@ -302,6 +302,8 @@ public:
     virtual void sizeChanged(int w, int h, int ow, int oh) = 0;
 };
 
+class HistogramObservable;
+
 /** This listener is used when the histogram of the final image has changed. */
 class HistogramListener
 {
@@ -334,6 +336,21 @@ public:
         const int waveformGreen[][256],
         const int waveformBlue[][256]
     ) = 0;
+    /** Tells which observable is notifying the listener. */
+    virtual void setObservable(HistogramObservable* observable) = 0;
+    /** Returns if the listener wants the histogram to be updated. */
+    virtual bool updateHistogram(void) = 0;
+    /** Returns if the listener wants the waveform to be updated. */
+    virtual bool updateWaveform(void) = 0;
+};
+
+class HistogramObservable
+{
+public:
+    /** Tells the observable to update the histogram data. */
+    virtual void updateHistogram() = 0;
+    /** Tells the observable to update the waveform data. */
+    virtual void updateWaveform() = 0;
 };
 
 /** This listener is used when the auto exposure has been recomputed (e.g. when the clipping ratio changed). */
diff --git a/rtgui/editorpanel.cc b/rtgui/editorpanel.cc
index 28747e063..5bf3e54d8 100644
--- a/rtgui/editorpanel.cc
+++ b/rtgui/editorpanel.cc
@@ -470,7 +470,8 @@ EditorPanel::EditorPanel (FilePanel* filePanel)
       iBeforeLockON (nullptr), iBeforeLockOFF (nullptr), previewHandler (nullptr), beforePreviewHandler (nullptr),
       beforeIarea (nullptr), beforeBox (nullptr), afterBox (nullptr), beforeLabel (nullptr), afterLabel (nullptr),
       beforeHeaderBox (nullptr), afterHeaderBox (nullptr), parent (nullptr), parentWindow (nullptr), openThm (nullptr),
-      selectedFrame(0), isrc (nullptr), ipc (nullptr), beforeIpc (nullptr), err (0), isProcessing (false)
+      selectedFrame(0), isrc (nullptr), ipc (nullptr), beforeIpc (nullptr), err (0), isProcessing (false),
+      histogram_observable(nullptr), histogram_scope_type(HistogramPanelListener::NONE)
 {
 
     epih = new EditorPanelIdleHelper;
@@ -2260,6 +2261,40 @@ void EditorPanel::histogramChanged(
     tpc->updateCurveBackgroundHistogram(histToneCurve, histLCurve, histCCurve, histLCAM, histCCAM, histRed, histGreen, histBlue, histLuma, histLRETI);
 }
 
+void EditorPanel::setObservable(rtengine::HistogramObservable* observable)
+{
+    histogram_observable = observable;
+}
+
+bool EditorPanel::updateHistogram(void)
+{
+    return histogram_scope_type == HistogramPanelListener::HISTOGRAM
+        || histogram_scope_type == HistogramPanelListener::NONE;
+}
+
+bool EditorPanel::updateWaveform(void)
+{
+    return histogram_scope_type == HistogramPanelListener::WAVEFORM
+        || histogram_scope_type == HistogramPanelListener::NONE;
+}
+
+void EditorPanel::scopeTypeChanged(ScopeType new_type)
+{
+    histogram_scope_type = new_type;
+
+    if (!histogram_observable) {
+        return;
+    }
+
+    // Make sure the new scope is updated since we only actively update the
+    // current scope.
+    if (new_type == HistogramPanelListener::HISTOGRAM) {
+        histogram_observable->updateHistogram();
+    } else if (new_type == HistogramPanelListener::WAVEFORM) {
+        histogram_observable->updateWaveform();
+    }
+}
+
 bool EditorPanel::CheckSidePanelsVisibility()
 {
     if (tbTopPanel_1) {
@@ -2376,6 +2411,10 @@ void EditorPanel::updateHistogramPosition (int oldPosition, int newPosition)
             break;
     }
 
+    if (histogramPanel) {
+        histogramPanel->setPanelListener(this);
+    }
+
     iareapanel->imageArea->setPointerMotionHListener (histogramPanel);
 
 }
diff --git a/rtgui/editorpanel.h b/rtgui/editorpanel.h
index 6a1fb585f..1794de11b 100644
--- a/rtgui/editorpanel.h
+++ b/rtgui/editorpanel.h
@@ -55,6 +55,7 @@ class EditorPanel final :
     public ThumbnailListener,
     public HistoryBeforeLineListener,
     public rtengine::HistogramListener,
+    public HistogramPanelListener,
     public rtengine::NonCopyable
 {
 public:
@@ -133,6 +134,12 @@ public:
         const int waveformGreen[][256],
         const int waveformBlue[][256]
     ) override;
+    void setObservable(rtengine::HistogramObservable* observable) override;
+    bool updateHistogram(void) override;
+    bool updateWaveform(void) override;
+
+    // HistogramPanelListener
+    void scopeTypeChanged(ScopeType new_type) override;
 
     // event handlers
     void info_toggled ();
@@ -265,4 +272,7 @@ private:
     bool isProcessing;
 
     IdleRegister idle_register;
+
+    rtengine::HistogramObservable* histogram_observable;
+    ScopeType histogram_scope_type;
 };
diff --git a/rtgui/histogrampanel.cc b/rtgui/histogrampanel.cc
index 8c2d971eb..d3d92880d 100644
--- a/rtgui/histogrampanel.cc
+++ b/rtgui/histogrampanel.cc
@@ -32,7 +32,7 @@ using namespace rtengine;
 //
 //
 // HistogramPanel
-HistogramPanel::HistogramPanel ()
+HistogramPanel::HistogramPanel () : panel_listener(nullptr)
 {
     setExpandAlignProperties(this, true, true, Gtk::ALIGN_FILL, Gtk::ALIGN_FILL);
     set_name("HistogramPanel");
@@ -355,12 +355,20 @@ void HistogramPanel::type_changed()
         showRAW->set_sensitive();
         showMode->set_sensitive();
         histogramRGBArea = histogramRGBAreaHori.get();
+        if (panel_listener) {
+            updateHistAreaOptions();
+            panel_listener->scopeTypeChanged(HistogramPanelListener::HISTOGRAM);
+        }
     } else {
         showValue->set_sensitive(false);
         showChro->set_sensitive(false);
         showRAW->set_sensitive(false);
         showMode->set_sensitive(false);
         histogramRGBArea = histogramRGBAreaVert.get();
+        if (panel_listener) {
+            updateHistAreaOptions();
+            panel_listener->scopeTypeChanged(HistogramPanelListener::WAVEFORM);
+        }
     }
 
     if (showBAR->get_active()) {
@@ -383,7 +391,8 @@ void HistogramPanel::bar_toggled ()
 void HistogramPanel::rgbv_toggled ()
 {
     // Update Display
-    histogramArea->updateOptions (showRed->get_active(), showGreen->get_active(), showBlue->get_active(), showValue->get_active(), showChro->get_active(), showRAW->get_active(), options.histogramDrawMode, options.histogramScopeType);
+    updateHistAreaOptions();
+    histogramArea->updateBackBuffer ();
     histogramArea->queue_draw ();
 
     histogramRGBArea->updateOptions (showRed->get_active(), showGreen->get_active(), showBlue->get_active(), showValue->get_active(), showChro->get_active(), showRAW->get_active(), showBAR->get_active());
@@ -441,6 +450,35 @@ void HistogramPanel::toggleButtonMode ()
         showMode->set_image(*mode2Image);
 }
 
+void HistogramPanel::setPanelListener(HistogramPanelListener* listener)
+{
+    panel_listener = listener;
+
+    if (listener) {
+        HistogramPanelListener::ScopeType type;
+        if (options.histogramScopeType == 0) {
+            type = HistogramPanelListener::HISTOGRAM;
+        } else {
+            type = HistogramPanelListener::WAVEFORM;
+        }
+        listener->scopeTypeChanged(type);
+    }
+}
+
+void HistogramPanel::updateHistAreaOptions()
+{
+    histogramArea->updateOptions(
+        showRed->get_active(),
+        showGreen->get_active(),
+        showBlue->get_active(),
+        showValue->get_active(),
+        showChro->get_active(),
+        showRAW->get_active(),
+        options.histogramDrawMode,
+        options.histogramScopeType
+    );
+}
+
 //
 //
 //
@@ -775,7 +813,7 @@ void HistogramRGBAreaVert::get_preferred_width_for_height_vfunc (int height, int
 //
 // HistogramArea
 HistogramArea::HistogramArea (DrawModeListener *fml) :
-    waveform_width(0),
+    waveform_width(0), wave_buffer_dirty(true),
     valid(false), drawMode(options.histogramDrawMode), myDrawModeListener(fml),
     scopeType(options.histogramScopeType),
     oldwidth(-1), oldheight(-1),
@@ -854,7 +892,7 @@ void HistogramArea::updateOptions (bool r, bool g, bool b, bool l, bool c, bool
     options.histogramDrawMode = drawMode   = mode;
     options.histogramScopeType = scopeType = type;
 
-    updateBackBuffer ();
+    wave_buffer_dirty = true;
 }
 
 void HistogramArea::update(
@@ -874,27 +912,31 @@ void HistogramArea::update(
 )
 {
     if (histRed) {
-        rhist = histRed;
-        ghist = histGreen;
-        bhist = histBlue;
-        lhist = histLuma;
-        chist = histChroma;
-        rhistRaw = histRedRaw;
-        ghistRaw = histGreenRaw;
-        bhistRaw = histBlueRaw;
-        waveform_scale = waveformScale;
-        if (waveform_width != waveformWidth) {
-            waveform_width = waveformWidth;
-            rwave.reset(new int[waveformWidth][256]);
-            gwave.reset(new int[waveformWidth][256]);
-            bwave.reset(new int[waveformWidth][256]);
+        if (scopeType == 0) {
+            rhist = histRed;
+            ghist = histGreen;
+            bhist = histBlue;
+            lhist = histLuma;
+            chist = histChroma;
+            rhistRaw = histRedRaw;
+            ghistRaw = histGreenRaw;
+            bhistRaw = histBlueRaw;
+        } else if (scopeType == 1) {
+            waveform_scale = waveformScale;
+            if (waveform_width != waveformWidth) {
+                waveform_width = waveformWidth;
+                rwave.reset(new int[waveformWidth][256]);
+                gwave.reset(new int[waveformWidth][256]);
+                bwave.reset(new int[waveformWidth][256]);
+            }
+            int (* const rw)[256] = rwave.get();
+            int (* const gw)[256] = gwave.get();
+            int (* const bw)[256] = bwave.get();
+            memcpy(rw, waveformRed, 256 * waveformWidth * sizeof(rw[0][0]));
+            memcpy(gw, waveformGreen, 256 * waveformWidth * sizeof(gw[0][0]));
+            memcpy(bw, waveformBlue, 256 * waveformWidth * sizeof(bw[0][0]));
+            wave_buffer_dirty = true;
         }
-        int (* const rw)[256] = rwave.get();
-        int (* const gw)[256] = gwave.get();
-        int (* const bw)[256] = bwave.get();
-        memcpy(rw, waveformRed, 256 * waveformWidth * sizeof(rw[0][0]));
-        memcpy(gw, waveformGreen, 256 * waveformWidth * sizeof(gw[0][0]));
-        memcpy(bw, waveformBlue, 256 * waveformWidth * sizeof(bw[0][0]));
         valid = true;
     } else {
         valid = false;
@@ -1185,31 +1227,35 @@ void HistogramArea::drawWaveform(Cairo::RefPtr<Cairo::Context> &cr, int w, int h
 
     // See Cairo documentation on stride.
     const int cairo_stride = Cairo::ImageSurface::format_stride_for_width(Cairo::FORMAT_ARGB32, waveform_width);
-    std::unique_ptr<unsigned char[]> buffer(new unsigned char[256 * cairo_stride]);
 
-    // Clear waveform.
-    memset(buffer.get(), 0, 256 * cairo_stride);
+    if (wave_buffer_dirty) {
+        wave_buffer.reset(new unsigned char[256 * cairo_stride]);
 
-    // TODO: Optimize.
-    for (int col = 0; col < waveform_width; col++) {
-        for (int val = 0; val < 256; val++) {
-            const float r = needRed ? scale * rwave[col][255 - val] : 0.f;
-            const float g = needGreen ? scale * gwave[col][255 - val] : 0.f;
-            const float b = needBlue ? scale * bwave[col][255 - val] : 0.f;
-            const float value = (r > g && r > b) ? r : ((g > b) ? g : b);
-            if (value <= 0) {
-                buffer[val * cairo_stride + col * 4 + 3] = 0;
-            } else {
-                buffer[val * cairo_stride + col * 4 + 3] = value;
-                buffer[val * cairo_stride + col * 4 + 2] = r;
-                buffer[val * cairo_stride + col * 4 + 1] = g;
-                buffer[val * cairo_stride + col * 4] = b;
+        // Clear waveform.
+        memset(wave_buffer.get(), 0, 256 * cairo_stride);
+
+        // TODO: Optimize.
+        for (int col = 0; col < waveform_width; col++) {
+            for (int val = 0; val < 256; val++) {
+                const unsigned char r = needRed ? scale * rwave[col][val] : 0;
+                const unsigned char g = needGreen ? scale * gwave[col][val] : 0;
+                const unsigned char b = needBlue ? scale * bwave[col][val] : 0;
+                const unsigned char value = (r > g && r > b) ? r : ((g > b) ? g : b);
+                if (value <= 0) {
+                    *(uint32_t*)&(wave_buffer[(255 - val) * cairo_stride + col * 4]) = 0;
+                } else {
+                    // Speedup with one memory access instead of four.
+                    *(uint32_t*)&(wave_buffer[(255 - val) * cairo_stride + col * 4]) =
+                        b | (g << 8) | (r << 16) | (value << 24);
+                }
             }
         }
+
+        wave_buffer_dirty = false;
     }
 
     Cairo::RefPtr<Cairo::ImageSurface> surface = Cairo::ImageSurface::create(
-        buffer.get(), Cairo::FORMAT_ARGB32, waveform_width, 256, cairo_stride);
+        wave_buffer.get(), Cairo::FORMAT_ARGB32, waveform_width, 256, cairo_stride);
     auto orig_matrix = cr->get_matrix();
     cr->translate(0, padding);
     cr->scale(static_cast<double>(w) / waveform_width, (h - 2 * padding) / 256.0);
diff --git a/rtgui/histogrampanel.h b/rtgui/histogrampanel.h
index f97d5188b..83099c68e 100644
--- a/rtgui/histogrampanel.h
+++ b/rtgui/histogrampanel.h
@@ -159,6 +159,8 @@ protected:
     int waveform_scale;
     int waveform_width;
     std::unique_ptr<int[][256]> rwave, gwave, bwave;
+    std::unique_ptr<unsigned char[]> wave_buffer;
+    bool wave_buffer_dirty;
 
     bool valid;
     int drawMode;
@@ -214,6 +216,14 @@ private:
     void get_preferred_width_for_height_vfunc (int height, int &minimum_width, int &natural_width) const override;
 };
 
+class HistogramPanelListener
+{
+public:
+    enum ScopeType {HISTOGRAM, WAVEFORM, NONE};
+
+    virtual void scopeTypeChanged(ScopeType new_type) = 0;
+};
+
 class HistogramPanel final : public Gtk::Grid, public PointerMotionListener, public DrawModeListener, public rtengine::NonCopyable
 {
 
@@ -255,9 +265,12 @@ protected:
     Gtk::Image *mode1Image;
     Gtk::Image *mode2Image;
 
+    HistogramPanelListener* panel_listener;
+
     sigc::connection rconn;
     void setHistInvalid ();
     void showRGBBar();
+    void updateHistAreaOptions();
 
 public:
 
@@ -304,4 +317,6 @@ public:
 
     // drawModeListener interface
     void toggleButtonMode () override;
+
+    void setPanelListener(HistogramPanelListener* listener);
 };