Yea, when tons of enemies are active under BrutalDoom, it causes a significant frame drop which worsens on the second bunch of enemies I encountered.
The map used is an WIP version of Oblige V4 (version 4.23 WIP) with the Nuts! + Crazy combo. I used it as an example to show the loss of framerate due to so many active monsters looking for the player.
Texture filter: Trilinear
Texture format: RGBA8
Vsync: On (60Hz)
Shader options: All enabled
Rendering quality: Quality
Sector light mode: Dark (Makes no difference)
Fog mode: Standard (Makes no diff in FPS drop)
Particle style: Smooth
Dynamic light options: All except additive lighting
Intensity: All the way to right
Light size: Near middle of scroll bar
AM2 3600+ Dual Core 1.91Ghz
ATI Radeon 3850 HD 512MB
Triple Buffering On (Made little difference if off)
It really shouldn't be dropping the framerate like this with settings like this, even at 640x480. This would definitely affect large megawads like Alien Vendetta and the Hell Revealed series.