add more caveats

src/docs/product/issues/issue-details/performance-issues/frame-drop/index.mdx

@@ -97,4 +97,8 @@ dispatch_apply(numberOfNumbers, dispatch_get_global_queue(QOS_CLASS_UTILITY, 0),
 });
 ```
 
-Keep in mind that concurrent solutions require analysis of thread safety for the code you're parallelizing. You may need to `@synchronize` critical sections, use semaphores, or dispatch back to a serial queue (or the main queue for UI work). Note that both `enumerateObjects...` and `dispatch_apply` are synchronous and won't return until all iterations have completed. Dispatch their invocation asynchronously off the main queue to avoid waiting.
+There are several things to Keep in mind when introducing concurrency:
+    - you may need to `@synchronize` critical sections, use semaphores, or dispatch back to a serial queue (or the main queue for UI work)
+    - you may not be able to parallelize loops whose iterations are dependent or where order is significant
+    - parallelization may not be more efficient for small collections, as [thread spawning has its own costs](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Multithreading/AboutThreads/AboutThreads.html#//apple_ref/doc/uid/10000057i-CH6-SW20); always measure first!
+    - both `enumerateObjects...` and `dispatch_apply` are synchronous and won't return until all iterations have completed. Dispatch their invocation asynchronously off the main queue to avoid waiting.