Startup a singleplayer game: lagspikes and rubberbanding, the game stutters immensly and framerate is 8-10 fps maximum. Create a new world: same situation. Try minimizing all graphical settings, including render distance set to 6 chunk: no luck either. load 1.15 just in case: works absolutely fine.

I tried also turning off smooth lighting to see weather it might be lighting-affected, but that didn't help. 

Hope this helps, I tried playing it for a while but it's impossibile, so I couldn't acquire any more "hard" data..!