Are you using custom items? If so, your items could be too high poly and are pulling your system to it's knees. If you're using the default items, unless you have a very slow CPU, you should be fine. Could you please state your CPU, RAM and Graphics card?
I've already installed the graphics drivers I used prior to the reinstall and I've tested changing the program's priority, no difference. Also changed render quality and such in the program to no avail.
Unless there's some obscure setting or driver I'm missing somewhere, this isn't making any sense...it worked perfectly fine before, so why not now?