I'd say it's all or nothing. If there was some damage to the processor you'd probably be experiencing system crashes or other things like that rather than a weird slowdown.
Sometimes, if a processor is damaged in just the right way, you'll be able to use it for some things, but not for others. A while ago I noticed that an AthlonXP I had around here would boot into an OS just fine, but any application that involved the use of floating point numbers(in this case it was the Folding@Home client) would crash almost immediately. I looked at the processor core and found a small chip in it that corresponded to where the floating point unit was located(or at least where most of it should have been.... ). Again though, that's the difference between the processor either working or not working at all, rather than a general slowdown across the board.
Do not meddle in the affairs of archers, for they are subtle and quick to anger.