This mod is beautiful. Unfortunately, if I have it installed (not necessarily actively blurring something) and I am in F11 fullscreen and I tab out, the computer becomes royally fucked up and almost unusable due to the slowness. The first time this happened I could barely open a tty to kill java.

I am absolutely sure it is Blur. I have tested it with only this single mod installed. There is no information in the console. Given that it only occurrs in fullscreen, could it be a bug in Minecraft? Do you have any idea what might cause this?

Maybe you could try using Fullscreen Windowed?

On Minecraft 1.8 fullscreen has some pretty big issues. In versions after 1.12 fullscreen works fine.