diff --git a/rtengine/impulse_denoise.h b/rtengine/impulse_denoise.h
index 0ae7e5b58..4d911201c 100644
--- a/rtengine/impulse_denoise.h
+++ b/rtengine/impulse_denoise.h
@@ -5,7 +5,7 @@
  *  it under the terms of the GNU General Public License as published by
  *  the Free Software Foundation, either version 3 of the License, or
  *  (at your option) any later version.
- * 
+ *
  *  RawTherapee is distributed in the hope that it will be useful,
  *  but widthITheightOUT ANY widthARRANTY; without even the implied warranty of
  *  MERCheightANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
@@ -15,7 +15,7 @@
  *  along with RawTherapee.  If not, see <http://www.gnu.org/licenses/>.
  *
  *  2010 Emil Martinec <ejmartin@uchicago.edu>
- *    
+ *
  */
 #include <cstddef>
 #include "rt_math.h"
@@ -29,17 +29,17 @@ using namespace std;
 namespace rtengine {
 
 void ImProcFunctions::impulse_nr (LabImage* lab, double thresh) {
-	
-	
+
+
 	// %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 	// impulse noise removal
 	// local variables
-	
+
 	int width = lab->W;
 	int height = lab->H;
-	
+
 	float hpfabs, hfnbrave;
-	
+
 	// buffer for the lowpass image
     float ** lpf = new float *[height];
 	// buffer for the highpass image
@@ -51,45 +51,57 @@ void ImProcFunctions::impulse_nr (LabImage* lab, double thresh) {
 		//memset (impish[i], 0, width*sizeof(unsigned short));
     }
 
-	
+
 	//The cleaning algorithm starts here
-	
+
 	//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 	// modified bilateral filter for lowpass image, omitting input pixel; or Gaussian blur
-	
+
 	static float eps = 1.0;
 	float wtdsum[3], dirwt, norm;
-	int i1, j1;	
-	
+	int i1, j1;
+
 	//rangeblur<unsigned short, unsigned int> (lab->L, lpf, impish /*used as buffer here*/, width, height, thresh, false);
-    #ifdef _OPENMP
-    #pragma omp parallel
-    #endif
+#ifdef _OPENMP
+#pragma omp parallel
+#endif
     {
 		AlignedBufferMP<double> buffer(max(width,height));
-	
+
 	    gaussHorizontal<float> (lab->L, lpf, buffer, width, height, max(2.0,thresh-1.0));
 	    gaussVertical<float>   (lpf, lpf, buffer, width, height, max(2.0,thresh-1.0));
     }
-	
+
 	//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-	
+
 	float impthr = max(1.0,5.5-thresh);
-	
+    float impthrDiv24 = impthr / 24.0f;	        //Issue 1671: moved the Division outside the loop, impthr can be optimized out too, but I let in the code at the moment
+
+#ifdef _OPENMP
+  #pragma omp parallel for private(hpfabs, hfnbrave,i1,j1)
+#endif
 	for (int i=0; i < height; i++)
 		for (int j=0; j < width; j++) {
-			
+
 			hpfabs = fabs(lab->L[i][j]-lpf[i][j]);
 			//block average of high pass data
 			for (i1=max(0,i-2), hfnbrave=0; i1<=min(i+2,height-1); i1++ )
 				for (j1=max(0,j-2); j1<=min(j+2,width-1); j1++ ) {
 					hfnbrave += fabs(lab->L[i1][j1]-lpf[i1][j1]);
 				}
-			hfnbrave = (hfnbrave-hpfabs)/24;
-			hpfabs>(hfnbrave*impthr) ? impish[i][j]=1 : impish[i][j]=0;
-			
+			impish[i][j] = (hpfabs>((hfnbrave-hpfabs)*impthrDiv24));
+
 		}//now impulsive values have been identified
-	
+
+// Issue 1671:
+// often, noise isn't evenly distributed, e.g. only a few noisy pixels in the bright sky, but many in the dark foreground,
+// so it's better to schedule dynamic and let every thread only process 16 rows, to avoid running big threads out of work
+// Measured it and in fact gives better performance than without schedule(dynamic,16). Of course, there could be a better
+// choice for the chunk_size than 16
+// race conditions are avoided by the array impish
+#ifdef _OPENMP
+  #pragma omp parallel for private(wtdsum,norm,dirwt,i1,j1) schedule(dynamic,16)
+#endif
 	for (int i=0; i < height; i++)
 		for (int j=0; j < width; j++) {
 			if (!impish[i][j]) continue;
@@ -110,17 +122,17 @@ void ImProcFunctions::impulse_nr (LabImage* lab, double thresh) {
 				lab->L[i][j]=wtdsum[0]/norm;//low pass filter
 				lab->a[i][j]=wtdsum[1]/norm;//low pass filter
 				lab->b[i][j]=wtdsum[2]/norm;//low pass filter
-			} 
-			
+			}
+
 		}//now impulsive values have been corrected
-	
+
     for (int i=0; i<height; i++) {
         delete [] lpf[i];
 		delete [] impish[i];
 	}
 	delete [] lpf;
 	delete [] impish;
-	
+
 }
 
 }