I would start asking checking into the possibility that the RAM upgrade is the culprit, rather than some more general bug in 2.0.

If you have installed the version of 2.0 that is designed to take advantage of the extra RAM (was that 2.01 or something?), then the problem might be that it's trying to fill its entire cache, and actually causing the lag you're experiencing.

Others here are more experienced with the tricks necessary to make the RAM upgrade perform properly for them, perhaps they can reply here...