Today, I've decided to do a quick comparison as to the difference in frames per second between Quake2World, R1Q2 with R1GL, and Alien Arena. This is not perfect or in-depth by any means, and does not represent the overall frames per second of each game. It was tested on the top edge of q2dm1, looking down at the tip of the lower age.
This is not precise: the settings are not all in-check (r1q2 is default settings, Alien Arena is low settings, Quake2World is default besides r_vertexbuffer 1); the locations are not 100% exact; all resolutions are at 1152x864 full-screen; no guns drawn on screen; field of view at 108; no anti-aliasing or anisotropic filtering.
Alien Arena was jumping rampantly from 333-500fps; the FPS counter probably isn't that accurate.
I hope to do a more in-depth comparison which includes things like dynamic lighting, more complex map geometry, and more in-depth model rendering. I also hope to do a comparison including more games that can load the Quake 2 map format. For now, lazy.