Two things...
#1... if you haven't already, you should do a clean install of nUI. This frame rate issue is very often related to old addon data and addon conflicts. Most of the people who report this problem find that it goes away if they do the clean install. Check
http://www.nUIaddon.com/documentation.html and click the "Installation" tab -- follow the instructions there for a first time installation.
#2... there is a slash command '/nui framerate {n}' command in nUI you can use to reduce how frequently nUI updates its screen elements. The default is '/nui framerate 30' which updates at 30fps. Reducing that to a lower number will cause some animations (such as the casting bar) to be less smooth, but can dramatically increase the system frame rate. But I would still do #1 above long before I tried just reducing the frame rate.