Just did some testing with my setup and also using a monitoring addon called OptionlessHouse. I left Optionlesshouse open through all tests so I know there was a slight hit to performance overall. I have 2 seperate WoW folders on my hdd. One has my original set of addons prior to using nUI, the 2nd has nUI and all the misc other addons I still use. I copied the WTF\config.wtf file from the nUI installed version to my original WoW directory as well to ensure video settings were identical.
My computer: Athlon XP Dual Core 2.0 GHz, 4 megs ram, 2 x 9800GT set in non SLI mode, 1680x1050 resolution.
Onto the fun stuff
Original before nUI UI:
In Ironforge solo:
Average FPS: 40
Mem/sec Minimum: 3.07 KiB
Mem/Sec Maximum: 7.107 KiB
Highest Mem/sec addon:Carbonite 6.83 KiB
Total addon only memory: 57.1 megs
In AV 40 Man:
Average FPS: 40
Mem/sec Minimum: 3.86 KiB
Mem/sec Maximum: 8.23 KiB
Highest Mem/sec addon:Carbonite 7.69 KiB
Total addon only memory: 57.2 megs
Current with nUI
In Ironforge solo:
Average FPS: 35
Mem/sec Minimum: 8.06 KiB
Mem/sec Maximum: 27.68 KiB
Highest Mem/sec addon: nUI 26.38 KiB
Total addon only memory:57.4 megs
In AV 40 Man:
Average FPS: 20
Mem/sec Minimum: 176.21 KiB
Mem/sec Maximum: 233.35 KiB
Highest Mem/sec addon: nUI 229.35 KiB
Total addon only memory: 60.8 megs
On both trials I ran the same route through IF and then through AV. In both AV runs I did not engage any enemies, nor get within viewable range of other players.
I'm not sure what all the numbers mean from a coding perspective though.
*edit: had the wrong resolution listed, lol