Typically it has to do with the GPU, because that actually renders what's on screen. But CPU and RAM also influence it indirectly. My desktop has a 650 with 24 gb ram and an FX8350. It runs just like any other game, that is, on Medium. It runs on Ultra on my laptop with a 970m (ram is identical and CPU is comparable). The ram improvement (from 8 to 24) didn't change anything, and the CPU load is relatively low during play