Compared to Skyrim or State of Decay, Rimworld don't need a good videocard.
Rimworld is basicly a big database with videooutput.
So the FPS don't based on the video it is based on CPU single core speed.
Mosttimes the issue are mods that give out error and slow down the game.
Be sure you have no error's after Rimworld startup before you started a colony, better don't use mods that give your errors.