Always keep in mind Windows is not a true multi tasking OS and P5 can require a lot of processing power. Switching to P5 when it has been running in background may not immediately show on the screen (when rendering or calculating a simulation). Be patient and P5 will show and respond. With only P5 running I just spend 20 minutes waiting for the each frame in a complex cloth simulation (after 20 minutes the frame number increses by 1 and the number after it changes, highest value up to now has been just over 2400). This is a very complex piece of cloth with (too) many collision objects, so don't panic, just wait.