I've had a similar problem while downloading files with the Firefox browser, on Mi Mix1 and to a somewhat lesser extent on Mi Note 2.
I e., the system becomes like syrup - everything works, but very slowly, and eventually I would get the "UI has stopped" message or similar.
I've had bigger problems with the apps Foldersync and Syncthings previously, locking up the system completely at times, but always assumed the problem was with these apps being resource hogs. Ok, Android should be able to handle apps like these also, and they never produced the "UI has stopped responding" message.
But I only recently - the previous two versions or so - have noticed this problem with Firefox.
I should add that everything else has worked very well, I've only gotten the syrupy effect while downloading in Firefox.