Hey! I have this minecraft bug since begining decembre. Every time i play minecaft in the version 1.14.x 1.15.x 1.16.x i watch at the top left every few seconds. I normaly play in the 1.8 and there is no problem. If i use a pvp client like badlion or lunar the bug is still there. This bug has nothing to do with my mouse because i normaly use a corsair harpoon wireless and switched to the roccat kone emp because my old mouse was broken and the bug is still there. (sorry for my bad english and here is a video from the bug )

I'm pretty sure that has nothing to do with Minecraft.

it has nothing to do with minecraft speedrunning

If im not mistaken, that motion blur is NOT a part of Vanilla Minecraft, unless you edited the video. Therefore, u could be using a mod in which case is not allowed in speedrunning in the first place.