Some things you can try (some of which you probably already have): turn off better lighting, turn off shadows, play in the lowest resolution, turn off the minimap, and zoom in as much as can and still play.
You could also play with the r_modelDetail and r_effectsDetail values in the console or cfg files. The UI will only let you get them so low. For both of them 0.0 is no detail and 1.0 is full detail.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, Din's Legacy, & Drox Operative 2