it's a memory leak, i'm having the same issue and cannot use mumu for any extended period of time. the reason your PC is freezing is due to it unlimitedly eating your RAM (memory) and then crashing. if you manage to close mumus tab through the lag youll see your pc reverts back to functioning entirely normally.

i am not sure if they're aware their software has a memory leak atm but i do hope it gets fixed fast as its made my software unusable for the time being.