diff --git a/NEWS b/NEWS
index f9ea105..afc8f62 100644
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,8 @@ Version ?.??.? (????-??-??):
    --iter-threshold are used together because -a inhibits -T
  * Improved performance when reading image data via standard input
  * Improved performance when using gray_stretch together with -g
+ * Improved performance when using option -p, --process-only together
+   with only the grayscale and/or mirror commands
  * Documentation improvements
 
 Version 2.24.1 (2024-12-11):
diff --git a/defines.h b/defines.h
index f14fa34..95ef818 100644
--- a/defines.h
+++ b/defines.h
@@ -172,6 +172,10 @@
 /* default luminance formula */
 #define DEFAULT_LUM_FORMULA REC709
 
+/* when to adapt threshold values to the image */
+#define INITIAL 0  /* adapt threshold unless is was already adapted */
+#define UPDATE 1   /* adapt threshold even if it was adapted before */
+
 /* foreground and background */
 typedef enum fg_bg_e {
   FG,
diff --git a/imgproc.c b/imgproc.c
index 8ccd31b..81a1eed 100644
--- a/imgproc.c
+++ b/imgproc.c
@@ -466,10 +466,13 @@ Imlib_Image make_mono(Imlib_Image *source_image, double thresh, luminance_t lt)
 
 /* adapt threshold to image values values */
 double adapt_threshold(Imlib_Image *image, double thresh, luminance_t lt,
-                       unsigned int flags)
+                       unsigned int flags, int force_update)
 {
   double t = thresh;
-  if(!(flags & ABSOLUTE_THRESHOLD)) {
+  static int is_adapted = 0;
+  if(is_adapted && !force_update) {
+    fprintf(stderr, "threshold is already adjusted to image\n");
+  } else if(!(flags & ABSOLUTE_THRESHOLD)) {
     if(flags & DEBUG_OUTPUT)
       fprintf(stderr, "adjusting threshold to image: %f ->", t);
     t = get_threshold(image, thresh/100.0, lt, 0, 0, -1, -1);
@@ -482,6 +485,7 @@ double adapt_threshold(Imlib_Image *image, double thresh, luminance_t lt,
       if(flags & DEBUG_OUTPUT)
         fprintf(stderr, " %f\n", t);
     }
+    is_adapted = 1;
   }
   if((flags & VERBOSE) || (flags & DEBUG_OUTPUT)) {
     fprintf(stderr, "using threshold %.2f\n", t);
diff --git a/imgproc.h b/imgproc.h
index a79809e..8f75d97 100644
--- a/imgproc.h
+++ b/imgproc.h
@@ -130,7 +130,7 @@ Imlib_Image crop(Imlib_Image *source_image, int x, int y, int w, int h);
 
 /* adapt threshold to image values values */
 double adapt_threshold(Imlib_Image *image, double thresh, luminance_t lt,
-                       unsigned int flags);
+                       unsigned int flags, int force_update);
 
 /* compute dynamic threshold value from the rectangle (x,y),(x+w,y+h) of
  * source_image */
diff --git a/ssocr.c b/ssocr.c
index 900e75b..efe2665 100644
--- a/ssocr.c
+++ b/ssocr.c
@@ -735,9 +735,6 @@ int main(int argc, char **argv)
                     min, max);
   }
 
-  /* adapt threshold to image */
-  thresh = adapt_threshold(&image, thresh, lt, flags);
-
   /* process commands */
   if(flags & VERBOSE) /* then print found commands */ {
     if(optind >= argc-1) {
@@ -771,6 +768,7 @@ int main(int argc, char **argv)
           n = 1;
           if(flags & VERBOSE) fputs(" processing dilation (1)\n", stderr);
         }
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = dilation(&image, thresh, lt, n);
         imlib_context_set_image(image);
         imlib_free_image();
@@ -790,6 +788,7 @@ int main(int argc, char **argv)
           n = 1;
           if(flags & VERBOSE) fputs(" processing erosion (1)\n", stderr);
         }
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = erosion(&image, thresh, lt, n);
         imlib_context_set_image(image);
         imlib_free_image();
@@ -809,6 +808,7 @@ int main(int argc, char **argv)
           n = 1;
           if(flags & VERBOSE) fputs(" processing opening (1)\n", stderr);
         }
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = opening(&image, thresh, lt, n);
         imlib_context_set_image(image);
         imlib_free_image();
@@ -828,18 +828,21 @@ int main(int argc, char **argv)
           n = 1;
           if(flags & VERBOSE) fputs(" processing closing (1)\n", stderr);
         }
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = closing(&image, thresh, lt, n);
         imlib_context_set_image(image);
         imlib_free_image();
         image = new_image;
       } else if(strcasecmp("remove_isolated",argv[i]) == 0) {
         if(flags & VERBOSE) fputs(" processing remove_isolated\n", stderr);
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = remove_isolated(&image, thresh, lt);
         imlib_context_set_image(image);
         imlib_free_image();
         image = new_image;
       } else if(strcasecmp("make_mono",argv[i]) == 0) {
         if(flags & VERBOSE) fputs(" processing make_mono\n", stderr);
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = make_mono(&image, thresh, lt);
         imlib_context_set_image(image);
         imlib_free_image();
@@ -860,6 +863,7 @@ int main(int argc, char **argv)
           if(flags & VERBOSE)
             fputs(" processing white_border (1)\n", stderr);
         }
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = white_border(&image, bdwidth);
         imlib_context_set_image(image);
         imlib_free_image();
@@ -874,6 +878,7 @@ int main(int argc, char **argv)
         }
         if(i+1<argc-1) {
           offset = (int) atoi(argv[++i]); /* sideeffect: increment i */
+          thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
           new_image = shear(&image, offset);
           imlib_context_set_image(image);
           imlib_free_image();
@@ -893,6 +898,7 @@ int main(int argc, char **argv)
         }
         if(i+1<argc-1) {
           mask = (int) atoi(argv[++i]); /* sideeffect: increment i */
+          thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
           new_image = set_pixels_filter(&image, thresh, lt, mask);
           imlib_context_set_image(image);
           imlib_free_image();
@@ -913,6 +919,7 @@ int main(int argc, char **argv)
         }
         if(i+1<argc-1) {
           mask = (int) atoi(argv[++i]); /* sideeffect: increment i */
+          thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
           new_image = keep_pixels_filter(&image, thresh, lt, mask);
           imlib_context_set_image(image);
           imlib_free_image();
@@ -935,6 +942,7 @@ int main(int argc, char **argv)
             fprintf(stderr, "\n");
           }
           i+=2; /* skip the arguments to dynamic_threshold */
+          thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
           new_image = dynamic_threshold(&image, thresh, lt, ww, wh);
           imlib_context_set_image(image);
           imlib_free_image();
@@ -946,30 +954,35 @@ int main(int argc, char **argv)
         }
       } else if(strcasecmp("rgb_threshold",argv[i]) == 0) {
         if(flags & VERBOSE) fputs(" processing rgb_threshold\n", stderr);
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = make_mono(&image, thresh, MINIMUM);
         imlib_context_set_image(image);
         imlib_free_image();
         image = new_image;
       } else if(strcasecmp("r_threshold",argv[i]) == 0) {
         if(flags & VERBOSE) fputs(" processing r_threshold\n", stderr);
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = make_mono(&image, thresh, RED);
         imlib_context_set_image(image);
         imlib_free_image();
         image = new_image;
       } else if(strcasecmp("g_threshold",argv[i]) == 0) {
         if(flags & VERBOSE) fputs(" processing g_threshold\n", stderr);
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = make_mono(&image, thresh, GREEN);
         imlib_context_set_image(image);
         imlib_free_image();
         image = new_image;
       } else if(strcasecmp("b_threshold",argv[i]) == 0) {
         if(flags & VERBOSE) fputs(" processing b_threshold\n", stderr);
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = make_mono(&image, thresh, BLUE);
         imlib_context_set_image(image);
         imlib_free_image();
         image = new_image;
       } else if(strcasecmp("invert",argv[i]) == 0) {
         if(flags & VERBOSE) fputs(" processing invert\n", stderr);
+        thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
         new_image = invert(&image, thresh, lt);
         imlib_context_set_image(image);
         imlib_free_image();
@@ -1000,6 +1013,7 @@ int main(int argc, char **argv)
             }
           }
           i+=2; /* skip the arguments to gray_stretch */
+          thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
           new_image = gray_stretch(&image, t1, t2, lt);
           imlib_context_set_image(image);
           imlib_free_image();
@@ -1033,6 +1047,7 @@ int main(int argc, char **argv)
             fprintf(stderr, "\n");
           }
           i += 4; /* skip the arguments to crop */
+          thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
           new_image = crop(&image, x, y, cw, ch);
           imlib_context_set_image(image);
           imlib_free_image();
@@ -1053,7 +1068,7 @@ int main(int argc, char **argv)
                             " (lum should be in [0,255])\n", min, max);
           }
           /* adapt threshold to cropped image */
-          thresh = adapt_threshold(&image, thresh, lt, flags);
+          thresh = adapt_threshold(&image, thresh, lt, flags, UPDATE);
         } else {
           fprintf(stderr, "%s: error: crop command needs 4 arguments\n", PROG);
           exit(99);
@@ -1068,6 +1083,7 @@ int main(int argc, char **argv)
             fprintf(stderr, "\n");
           }
           theta = atof(argv[++i]); /* sideeffect: increment i */
+          thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
           new_image = rotate(&image, theta);
           imlib_context_set_image(image);
           imlib_free_image();
@@ -1117,6 +1133,9 @@ int main(int argc, char **argv)
   /* exit if only image processing shall be done */
   if(flags & PROCESS_ONLY) exit(3);
 
+  /* adapt threshold to image (unless this is already done) */
+  thresh = adapt_threshold(&image, thresh, lt, flags, INITIAL);
+
   if(flags & USE_DEBUG_IMAGE) {
     /* copy processed image to debug image */
     debug_image = make_mono(&image, thresh, lt);