It replaces episode 3 in Ultimate Doom as a pwad, for the sake of utilising the ending sequences I believe, so not for Doom II.
I am quite surprised at some of the performance issues. On one hand, I understand nobody would be playing this on a 486 (except maybe the Nostalgia Nerd) and people wouldn't even notice, but on the other hand I expected it to be on-par with the original game in terms of performance. I guess I'm nitpicking, but I figured it would have been an important factor from the guy who made most of those original levels. At least on-par with TNT from Final Doom, which had pretty bad performace in some maps. I don't know, it's a non-issue if most people are playing it on a Ryzen Threadripper, but it does bother me anyway. Maybe I would have liked to play it with DSDoom. (*which the NDS could not handle a complete playthrough (of Final Doom) due to poor map optimisation-related performance issues)