Note that the Geforce series of cards are not officially supported by SOLIDWORKS.
But, see this thread which appears to be the same issue:
If you look through it, you'll see that the problem appears to be with updates NVIDIA made to their graphics drivers. At the time I'm writing this, it looks like NVIDIA knows about the problem and has made some improvements in their latest drivers, but one person states that installing an older driver has even better performance. You'll have to read the thread for the latest details since it is very active and things may continue to change rapidly as NVIDIA updates drivers or people discover other solutions.
Thanks for the link! It seems like I'm having that same issue. I backdated my driver to 368.69 and the difference is massive. Still not perfect, but about 80% of the way there and the best performance I've seen on a non-Firepro/Quadro machine. It's usable now. Looks like I'll join those folks in sitting around waiting for a solution from nVidia.