Well usually I run CSS at 300 fps (capped) with 10% cpu for hl2.exe Sometimes it just goes up to 100% and the whole computer is very slow. Mouse is slow, unresponsive windows explorer, etc. In order to fix it, I would have to restart my computer.
Also when I close CSS directly (without pressing the Quit button) [The Red X on the top right] The window would close, but the process would stay alive and take 50% which half the time I don't even know that it is running.