oneself, there should be no performance penalty related to the number of completed tasks. If you do notice anything being slow, please let us know -- we're always trying to make RTM as fast as possible :)

To remove clutter, I have taken to archiving non-repeating completed tasks periodically. I have a smart list that shows me the candidates, and then I move them over to a cold storage list which I archive.

Of all the things I ask RTM to do, moving many tasks at once like this is one of the hardest, with a long delay and repeated failures before I have everything squared away. Since I only do this a few times a year, I've never worried about that, but I do like flushing the old stuff out of the active lists.