I have actually gone through the whole tedious process of a 'clean install' twice already. I use a ton of mods and usually wind up around 100megs of addon use so I am guessing that's my problem. But I have had an overall major decrease to my fps with nUI. I love the addon and will not change cuz it's just that awsome, but I do wish I could figure out a way around it. I have tried nUI with the clean install without some of the less 'necessary' addons but my fps still tanked. One interesting note to all this is that I only really get the fps crapout in LK areas. When I play my lower alts, even when grouping and being run through an instance I don't have the loss of fps as I do when I'm on my main in Dalaran and running heroics. Not sure if that helps any, and if you need any extra info let me know.
On a side note the very first clean install I did I ran it exactly and Scott mentioned. Got rid of everything then added nUI and then all my other addons one by one. At first my fps was 'okay' (didn't absolutely tank but was not as good as no addons at all) and I got all the way through the clean install with my fps not dropping below 20 fps and that was in Dalaran running around in circles and juming all over the place on my epic land mount. But then later that night or the next day, I don't remember exactly, my fps tanked back below 10 fps at times when raiding and running in Dalaran.
If you need any specific info such as the addons I use or anything like that let me know and I can post my WowMatrix list here and anything else that might be helpful to all those super computer geeks out there.
