All the MutationObserver stuff in TextInputWidget, which calls #adjustSize for autosized inputs after they're inserted into the document, is slow. There is some overhead for creating the MutationObserver, it (probably) slows down DOM operations a little bit, the handler for it is executed with some delay, and when building an interface with many TextInputWidgets, each of them runs separately and causes a separate reflow when #adjustSize is called. We really should find a way to do this less often, or not at all.
|T71799 Editing a transclusion with many templates hangs the browser
|T134814 Multi-part transclusion dialog a little slow to load
|T135085 Increase the limit of 50 uploads at once (maxUploads)
|T153210 Make upload interface changes a little snappier
|T154024 Kill slow MutationObserver stuff in TextInputWidget
|T176998 Try using SentinelJS (or stealing code from it) instead of nasty code in TextInputWidget#installParentChangeDetector