Just jumping in to say I ran into this problem over the weekend as well. Stepped away from my PC for maybe half an hour, and when I came back it was unusably slow. Programs took forever to launch or wouldn't launch at all, Chrome eventually launched but wouldn't load anything, etc. When I got the Task Manager to open (which took about 5 minutes), the only thing out of the ordinary was that the Malwarebytes Service was pegged using 25% of my CPU. Normally it uses <1%. I was unable to get MWB to open, so I attempted a reboot which got stuck and had to force a shutdown.
On reboot I disabled the Ransomware Protection module and that seems to have fixed things. By which I mean the problem hasn't reoccurred in the past couple of days.
This is with MWB v3.4.5 on Windows 10. Like PowerMan, my PC's reasonably fast, though not as powerful as his, so this kind of behavior was extremely unusual and seems to have come from MWB going a bit rogue. I'm happy to start my own thread / gather diagnostic logs if need be.