Step #1 -- if you haven't yet, try following the steps in this FAQ as that alone normally resolves a lot of FPS issues for folks...
http://www.wowinterface.com/forums/s...ad.php?t=21716
Step #2 -- reducing shadow detail seems to seriously help a lot of people's FPS without impacting spell detail, etc.
Step #3 -- If you don't need the full 25 man raid panel while in combat (i.e... you're not trying to heal the raid) then use '/nui autogroup' to turn automatic panel switching off and manually switch your unit frame panel to the solo mode or party mode
I am aware that some people are having FPS issues with the raid panels and that is one of the key issues I am attempting to tackle with nUI 6.0