Mako: It's very easy to filter only the backgrounds and leave out the 3D, that was never a problem. The problem is that you can't simply filter the background and get a good result, no matter how you do it.
FPS issues are usually due to misconfiguration, like trying to do scaling/postprocessing on a crappy onboard graphics card. It's a very very taxing operation on a card that doesn't have it's own dedicated memory, no matter how you do it. Or even worse, missing/not having access to a useful OpenGL driver altogether, Vista and later like to revert to their own software renderer instead of bailing out, with disastrous results.