I'm using Solidworks 2011 and have found that there is a noticable lag while tabbing between open Solidworks documents. I can count to 4 or 5 before the window changes. With 2010 there was no lag.
My system is XP Pro with all updates (I know...) running on a dell precision 380 with 4gig of ram and an Nvidia Qudra FX540.
Sounds like a video card/driver issue. That is a very old video card.
1. Make sure you are using the latest approved driver, found here: http://www.solidworks.com/sw/support/videocardtesting.html
2. After installing the latest video driver, make sure to go into the Advanced 3D settings. Change it to SolidWorks default. Also, make sure antialiasing is turned OFF. Also, make sure vertical sync (v-sync) is turned OFF. These two settings can cause the issue you described.