I get pretty low fps counts with levels that go far beyond what vanilla Doom was meant to. The latest map I'm having problems with is The Spire 2 (also requires gothictx.wad) so I'll use it as a test case.
With GLBoom+, looking up from the starting point at the large tower (as in the screenshot), I get about 55 fps.
With GZDoom under my "normal" settings (1024x768, dynamic lights on but shaders for lights off, most other things maxed out) I get ~7 fps.
If I disable fog and set the lighting mode from Software to Standard, I get up to ~14 fps.
I saw somewhere a command-line switch -sm2 (disables shaders entirely I think?), using that I can get a tolerable 21 fps. However the framerate is quite unsteady, moreso than normal. Can this be set in the options or zdoom.ini anywhere?
Any of the other settings I've tried have no effect (or maybe 1 fps combined) on the fps, including texture filtering, AA, screen resolution, HQ resize, etc., which makes me think there's a bottleneck somewhere.
Anyhow I'm sure you're quite sick of mystery ATI card problems by now, just wondering if there's a quick fix I haven't tried yet.
In case it matters: logfile
Spoiler:screenshots of normal and -sm2 rendering, with stat renderstats and stat rendertimes