That is very interesting. I play Deus Ex with New Vision, and the HDTP (High Definition Texture Pack) plus the EBN Series mod and Direct X 10 support patch (they have DX 9 support mods too) on an ATI Readon 5750 1 gig ddr 5 card with no game speed problems. Seems to me if anycard should speed up a game this is one of them. I wonder what causes such things to happen.
That's the cool thing about having all those mods and different renderers for Deus Ex. You actually can have a choice of 4 different ones to play the game. The Default, DX9, DX10, and Open GL. All have their good points and bad points. You might want to try other texture or high definition packs with different renderers to see if the video style is to your liking and perhaps one of those other rendeerers will solve your problem.
Here is a page with info on all those mods and renderers:
http://thenamelessmod.com/wiki/TNM_HD
Also get The Nameless Mod - Its the super awesome largest total conversion mod, 7 years in the making that there is for Deus Ex. It's like 2 full games in one so it will be like you have 3 great Deus Ex games. Links to it also in that wiki.
I have also read on the Net that New Vision works with Direct X 10 the renderer. You may want to get the DX 10 and give it a try. (I forget which on I am currently using- my game is installed but I haven't touched it for a while.)
Hey, if that don't work, try posting over in the TNM Forums and they might be able to help cus those folks use the New Vision Mod a lot in their TC.