Main thread appears to be blocked
If you're using the Chrome Developer Tools (i.e., DevTools) or the Safari Web Inspector to profile the performance of your app, you may notice asynchronous XMLHttpRequest sends appear to become blocking, as if they were being made synchronously. This behavior is a known issue with the DevTools. The blocking behavior does not manifest itself with the DevTools closed, nor when using the Firefox Developer Tools.
Updated over 4 years ago