Tweaked wording for v8 "performance cliff" issue

Author: Brian Vaughn

packages/react-reconciler/src/ReactFiber.js

@@ -254,14 +254,16 @@ function FiberNode(
   this.alternate = null;
 
   if (enableProfilerTimer) {
-    // Note: The following is done to avoid a v8 deopt.
+    // Note: The following is done to avoid a v8 performance cliff.
     //
-    // It is important to initialize the fields below with doubles.
-    // Otherwise Fibers will deopt and end up having separate shapes when
-    // doubles are later assigned to fields that initially contained smis.
-    // This is a bug in v8 having something to do with Object.preventExtension().
+    // Initializing the fields below to smis and later updating them with
+    // double values will cause Fibers to end up having separate shapes.
+    // This behavior/bug has something to do with Object.preventExtension().
+    // Fortunately this only impacts DEV builds.
+    // Unfortunately it makes React unusably slow for some applications.
+    // To work around this, initialize the fields below with doubles.
     //
-    // Learn more about this deopt here:
+    // Learn more about this here:
     // https://github.com/facebook/react/issues/14365
     // https://bugs.chromium.org/p/v8/issues/detail?id=8538
     this.actualDuration = Number.NaN;
@@ -270,7 +272,8 @@ function FiberNode(
     this.treeBaseDuration = Number.NaN;
 
     // It's okay to replace the initial doubles with smis after initialization.
-    // This simplifies other profiler code and doesn't trigger the deopt.
+    // This won't trigger the performance cliff mentioned above,
+    // and it simplifies other profiler code (including DevTools).
     this.actualDuration = 0;
     this.actualStartTime = -1;
     this.selfBaseDuration = 0;