Configurable SLOWDOWN divisor (#17171)

Marlin/Configuration_adv.h

 // Minimum time that a segment needs to take if the buffer is emptied
 #define DEFAULT_MINSEGMENTTIME        20000   // (ms)
 
-// If defined the movements slow down when the look ahead buffer is only half full
+// Slow down the machine if the look ahead buffer is (by default) half full.
+// Increase the slowdown divisor for larger buffer sizes.
 #define SLOWDOWN
+#if ENABLED(SLOWDOWN)
+  #define SLOWDOWN_DIVISOR 2
+#endif
 
 // Frequency limit
 // See nophead's blog for more info

Marlin/src/module/planner.cpp

   #endif
 
   #if ENABLED(SLOWDOWN)
-    if (WITHIN(moves_queued, 2, (BLOCK_BUFFER_SIZE) / 2 - 1)) {
+    #ifndef SLOWDOWN_DIVISOR
+      #define SLOWDOWN_DIVISOR 2
+    #endif
+    if (WITHIN(moves_queued, 2, (BLOCK_BUFFER_SIZE) / (SLOWDOWN_DIVISOR) - 1)) {
       if (segment_time_us < settings.min_segment_time_us) {
         // buffer is draining, add extra time.  The amount of time added increases if the buffer is still emptied more.
         const uint32_t nst = segment_time_us + LROUND(2 * (settings.min_segment_time_us - segment_time_us) / moves_queued);