I checked the recent build 2889. The gains in game speed due to the latest committed fixes are freakin' awesome. Now I get three times the FPS compared to older builds. Wow! Here's hope that more complex maps will finally be playable in Doomsday, but I haven't tried that yet due to the following bug:
The keybinding system for the weapon slots is broken. Instead of selecting a particular weapon bound to a certain key, all available weapons are cycled through when repeatedly pressing the same weapon slot key. Doom, Heretic and Hexen are affected likewise.