Hello mojang, Jankowski here (Poland).
Before i'll say something else, i just wanna say, this is different bug, than mouse speed up. Its like this, but inverse...

So, i was playing minecraft few days ago, my mouse was perfectly working
Then, i re-opened minecraft next day, and boom, mouse is lagging. I dont know why or how it suddenly started to behavior like that. It was just a simple computer turn off, and then turn on next day!

I wasn't doing any stuff with my mouse, its working perfectly on any other applications.
Didnt install any sh*t, didnt uninstall any... you know what . It just started to slow down randomly, propabally mouse broke during the night (Yeach i have nice imagination).

Ah, by the way im using MultiMC5, but i whink that's not the problem. Mouse was working perfectly with this launcher for 1 year.

If you think its not a minecraft bug, then idk maybe aliens arived at night and hammered my mouse or smth.