Source is a limited engine, but I have to applaud the level of optimization. Portal 2 (or even better, HL2) will run fine even on really stuffty hardware. Unity and Unreal are nowhere near as fast, and while I have a decent computer now, Valve games and GMod were some of the only 3D games I could play on my laptop. KSP had single-digit FPS before even launching a rocket, while Portal 2 would run at 60fps on decent graphical settings.