[Narishma]

It's just poorly coded and doesn't take advantage of the CPU power on hand. A lot of games are like that, both indie and AAA.

[Der_Einzige]

Poorly coded? RimWorld handles hundreds of mods mostly gracefully. Combine it with rimpy GPU compiling textures and you shouldn't have fps issues unless your computer is a potato or you have tons of VRAM heavy mods.