diff --git a/wled00/FX_2Dfcn.cpp b/wled00/FX_2Dfcn.cpp
index 751bc5ac14..063d3a6bb3 100644
--- a/wled00/FX_2Dfcn.cpp
+++ b/wled00/FX_2Dfcn.cpp
@@ -17,6 +17,7 @@
 // note: matrix may be comprised of multiple panels each with different orientation
 // but ledmap takes care of that. ledmap is constructed upon initialization
 // so matrix should disable regular ledmap processing
+// WARNING: effect drawing has to be suspended (strip.suspend()) or must be called from loop() context
 void WS2812FX::setUpMatrix() {
 #ifndef WLED_DISABLE_2D
   // isMatrix is set in cfg.cpp or set.cpp
@@ -45,12 +46,12 @@ void WS2812FX::setUpMatrix() {
       return;
     }
 
-    suspend();
-    waitForIt();
-
     customMappingSize = 0; // prevent use of mapping if anything goes wrong
 
     d_free(customMappingTable);
+    // Segment::maxWidth and Segment::maxHeight are set according to panel layout
+    // and the product will include at least all leds in matrix
+    // if actual LEDs are more, getLengthTotal() will return correct number of LEDs
     customMappingTable = static_cast<uint16_t*>(d_malloc(sizeof(uint16_t)*getLengthTotal())); // prefer to not use SPI RAM
 
     if (customMappingTable) {
@@ -113,7 +114,6 @@ void WS2812FX::setUpMatrix() {
 
       // delete gap array as we no longer need it
       p_free(gapTable);
-      resume();
 
       #ifdef WLED_DEBUG
       DEBUG_PRINT(F("Matrix ledmap:"));
diff --git a/wled00/FX_fcn.cpp b/wled00/FX_fcn.cpp
index 2d98dc0447..39f87434c1 100644
--- a/wled00/FX_fcn.cpp
+++ b/wled00/FX_fcn.cpp
@@ -1681,9 +1681,13 @@ void WS2812FX::setTransitionMode(bool t) {
   resume();
 }
 
-// wait until frame is over (service() has finished or time for 1 frame has passed; yield() crashes on 8266)
+// wait until frame is over (service() has finished or time for 2 frames have passed; yield() crashes on 8266)
+// the latter may, in rare circumstances, lead to incorrectly assuming strip is done servicing but will not block
+// other processing "indefinitely"
+// rare circumstances are: setting FPS to high number (i.e. 120) and have very slow effect that will need more
+// time than 2 * _frametime (1000/FPS) to draw content
 void WS2812FX::waitForIt() {
-  unsigned long maxWait = millis() + getFrameTime() + 100; // TODO: this needs a proper fix for timeout!
+  unsigned long maxWait = millis() + 2*getFrameTime() + 100; // TODO: this needs a proper fix for timeout! see #4779
   while (isServicing() && maxWait > millis()) delay(1);
   #ifdef WLED_DEBUG
   if (millis() >= maxWait) DEBUG_PRINTLN(F("Waited for strip to finish servicing."));
@@ -1810,7 +1814,11 @@ Segment& WS2812FX::getSegment(unsigned id) {
   return _segments[id >= _segments.size() ? getMainSegmentId() : id]; // vectors
 }
 
+// WARNING: resetSegments(), makeAutoSegments() and fixInvalidSegments() must not be called while
+// strip is being serviced (strip.service()), you must call suspend prior if changing segments outside
+// loop() context
 void WS2812FX::resetSegments() {
+  if (isServicing()) return;
   _segments.clear();          // destructs all Segment as part of clearing
   _segments.emplace_back(0, isMatrix ? Segment::maxWidth : _length, 0, isMatrix ? Segment::maxHeight : 1);
   _segments.shrink_to_fit();  // just in case ...
@@ -1818,6 +1826,7 @@ void WS2812FX::resetSegments() {
 }
 
 void WS2812FX::makeAutoSegments(bool forceReset) {
+  if (isServicing()) return;
   if (autoSegments) { //make one segment per bus
     unsigned segStarts[MAX_NUM_SEGMENTS] = {0};
     unsigned segStops [MAX_NUM_SEGMENTS] = {0};
@@ -1889,6 +1898,7 @@ void WS2812FX::makeAutoSegments(bool forceReset) {
 }
 
 void WS2812FX::fixInvalidSegments() {
+  if (isServicing()) return;
   //make sure no segment is longer than total (sanity check)
   for (size_t i = getSegmentsNum()-1; i > 0; i--) {
     if (isMatrix) {
@@ -1951,6 +1961,7 @@ void WS2812FX::printSize() {
 
 // load custom mapping table from JSON file (called from finalizeInit() or deserializeState())
 // if this is a matrix set-up and default ledmap.json file does not exist, create mapping table using setUpMatrix() from panel information
+// WARNING: effect drawing has to be suspended (strip.suspend()) or must be called from loop() context
 bool WS2812FX::deserializeMap(unsigned n) {
   char fileName[32];
   strcpy_P(fileName, PSTR("/ledmap"));
@@ -1980,9 +1991,6 @@ bool WS2812FX::deserializeMap(unsigned n) {
   } else
     DEBUG_PRINTF_P(PSTR("Reading LED map from %s\n"), fileName);
 
-  suspend();
-  waitForIt();
-
   JsonObject root = pDoc->as<JsonObject>();
   // if we are loading default ledmap (at boot) set matrix width and height from the ledmap (compatible with WLED MM ledmaps)
   if (n == 0 && (!root[F("width")].isNull() || !root[F("height")].isNull())) {
@@ -2040,8 +2048,6 @@ bool WS2812FX::deserializeMap(unsigned n) {
     DEBUG_PRINTLN(F("ERROR LED map allocation error."));
   }
 
-  resume();
-
   releaseJSONBufferLock();
   return (customMappingSize > 0);
 }
diff --git a/wled00/set.cpp b/wled00/set.cpp
index 893081b986..baa2f305a7 100644
--- a/wled00/set.cpp
+++ b/wled00/set.cpp
@@ -815,8 +815,13 @@ void handleSettingsSet(AsyncWebServerRequest *request, byte subPage)
       }
     }
     strip.panel.shrink_to_fit();  // release unused memory
+    // we are changing matrix/ledmap geometry which *will* affect existing segments
+    // since we are not in loop() context we must make sure that effects are not running. credit @blazonchek for properly fixing #4911
+    strip.suspend();
+    strip.waitForIt();
     strip.deserializeMap(); // (re)load default ledmap (will also setUpMatrix() if ledmap does not exist)
     strip.makeAutoSegments(true); // force re-creation of segments
+    strip.resume();
   }
   #endif