nUI shouldn't have any effect on your latency. Latency is a measurement of your connection speed and as far as I'm aware, nUI has no control over your net connection to the WoW servers.
Leatrix has a couple of addons that do seem to help some people though. They may or may not help you, but it's always worth a shot.
Leatrix Latency Fix will change some settings
in the computers operating systemthat many have stated improve their, you guessed it, latency. It is fully reversible, but make sure to read everything on the addon page before deciding to apply it.
http://www.wowinterface.com/download...atencyFix.html
To help you with framerates, you can manually tweak your video option through the in game Video settings and reduce things like shadows, view distance etc. You can also try another of Leatrix's addons named Leatrix Gfx. That addon will allow you to change some other settings that would normally require you to edit the config.wtf file.
http://www.wowinterface.com/download...eatrixGfx.html
And just for reference, what is your OS and your computer system specs?
***EDIT***
Added bolded/italic statement wording to clarify what Leatrix Latency fix affects