IF does in fact run on normal eduke32. Somewhat paradoxically, it reportedly runs better in software mode as long as you set a low enough resolution. My guess is GL mode runs worse because it uses a lot of blending operations and Polymost uses occlusion queries to determine portal visibility. And with such detalization there is a shitton of portals.
You can also try disabling on disk texture caching.
You can also try disabling on disk texture caching.