If you are having performance issues with Plasma, it's most likely that your graphics card driver has issues with the fancy hotness known cryptically as "ARGB visuals". In plain terms, they let us do fancy translucency. Without them, you still get a nicely composited desktop, but the panel will not be translucent, popups will look a bit more plain and you won't get all the hot sizzle you deserve from Plasma.
Still, for some, that's the only option. Today I was talking with a fellow on IRC who manages thin client KDE deployments in school environments, and they have the challenge of old hardware for which there is no realistic chance of new and improved drivers appearing. It occurred to me that we ought to have an environment variable for this.
When I opened up the appropriate source file, I was delighted to see Seli had already stumbled upon this idea and implemented it. Huzzah for open source!
So for those in such situations, you can selectively turn off ARGB visuals when Plasma starts simply by ensuring that the KDE_SKIP_ARGB_VISUALS environment variable is set to the value "1" (without the quotes, obviously ;) and life will be slightly sweeter for you, if a little less pretty.
This appeared during the 4.2 dev cycle, so KDE 4.1 users are probably out luck ... but you wanted 4.2 anyways, right? ;)