Some games freeze my system completely

I've never seen that happen on on ne PC's because UI usually runs in separate thread so no freezing on multicore CPUs. (Though insane lagging/ constant no response is still possible)

Just a note here: Threads have nothing to do with CPU cores, threads & processes are OS specific... Single core can run multiple threads, so no freezing on single core either (with a proper OS).

The point is that the OS scheduler prioritizes one process' threads over others, including the OS' UI thread. Unless the process' threads idle, other threads on the run queue will be starved of CPU time.