The largest performance hits associated with the UI that I've found are:
1) Massive tileling - Can't be completely avoided as frames are prety much required to accomodate variable sized windows, but minimizing use of frames is helpfull.
2) Massive numbers of resource files - the default UI is horible about this. UI graphics are spread across a rediculious number of files. In theory the client only loads images slices as needed but in practice having all the slices come from a small number of DDS files helps.
3) Complex Alphas - Another thing the default UI is bad about. No transparency images are not only smaller in memory but take less processing to overlay atop the game environment. The default UI is particularly bad in that it uses the most pigish format for virtually everything. 1 bit alphas aren't much of an issue, but multi-bit alphas can have a noticable impact.
At one point I developed a fancy version of the Knowledge window that made use of a tiled background with a complex alpha channel. Opening that one window noticebly lagged the game for about 30 seconds and performance suffered while it was open. I replaced the tiling frame with a presized opaque image and the lag vanished.
Smaller viewscreen size does improve performance, you just might not notice the difference if your lag stems from other rendering issues. Ideally, all your UI windows would be place outside the game area. Realistically that's not gonna happen, but you can place most of the "Always on" windows off the game area.
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