I did manage to find a fix after playing around with different settings. After thinking about it a little more, I thought that maybe this all started right when I updated with the June 2019 Windows update. Sure enough, "Game Mode" was turned back on on my computer. I turned it off and I also disabled fullscreen optimizations, and everything went back to normal. It's as if something was preventing my system from dedicating the appropriate resources to XIV. If you haven't already, these might be things to check? I hope you can find a fix on your end.