limit pulsing blocks animation frame rate to 30FPS

possibly resolves #1941

frontend/src/app/components/blockchain-blocks/blockchain-blocks.component.scss

@@ -114,7 +114,11 @@
 }
 
 .flashing {
-  animation: opacityPulse 2s ease-out;
+  /* force compositing */
+  will-change: opacity;
+  transform: translateZ(0);
+  /* effective max frame rate = (#keyframes - 1) x steps / duration */
+  animation: opacityPulse 2s steps(30, end);
   animation-iteration-count: infinite;
   opacity: 1;
 }

frontend/src/app/components/mempool-blocks/mempool-blocks.component.scss

@@ -17,7 +17,11 @@
 }
 
 .flashing {
-  animation: opacityPulse 2s ease-out;
+  /* force compositing */
+  will-change: opacity;
+  transform: translateZ(0);
+  /* effective max frame rate = (#keyframes - 1) x steps / duration */
+  animation: opacityPulse 2s steps(30, end);
   animation-iteration-count: infinite;
   opacity: 1;
 }