Fabric: Stop preallocation views on the main thread

Summary:
There is no reason to allocate views ahead of time on the main thread.

There is a chance that this view will not be mounted and we are not saving any time because it's a sequential process anyway (because we are doing it on the main thread). Moreover, the switching context can only slowdown JS execution.

Reviewed By: JoshuaGross

Differential Revision: D14019433

fbshipit-source-id: 83ac05a91e4b70cb382a55d6687752480984404e

React/Fabric/Mounting/RCTMountingManager.mm

@@ -178,6 +178,11 @@ using namespace facebook::react;
 
 - (void)optimisticallyCreateComponentViewWithComponentHandle:(ComponentHandle)componentHandle
 {
+  if (RCTIsMainQueue()) {
+    // There is no reason to allocate views ahead of time on the main thread.
+    return;
+  }
+
   RCTExecuteOnMainQueue(^{
     [self->_componentViewRegistry optimisticallyCreateComponentViewWithComponentHandle:componentHandle];
   });