because inspect tab checks for loads of variables that have to be refreshed each tick, therefore the slowdown doesn't happen because of number of objects / what you did on the scene, but because debugger has to update lots of values.
it's a good thing that you can add to watch variables that only you are interested into, and change tab, which then shows the real performance. ofcourse if you watch too many vars you can get the same slowdown on watch tab too i think best to check fps/cpu/etc is to create your own ingame debugger and debug with it, turnable by some letter / variable