Reloading a resource pack enough times will cause intense FPS stuttering. This occurs very consistently with a pack that has a big enough atlas (I believe). I have attached a video and a demo resource pack showing this issue. It is very detrimental to resource pack creation and the only way to temporarily fix is to restart the game. The generated atlases include one which is 8192x8192. I attached a screenshot of the atlas sizes as well.
After reloading about 8 times, my game was at under 2 fps and the fps tracker was solid red (see attached screenshot).
To reproduce, load the attached resource pack, create a void world, and then keep reloading. It should get unplayable within about 5 reloads.