The importance of frame rate
Drops in frame rate are much more acceptable outside of VR than in VR; if a computer is good enough to run a game at 30 frames a second but not at 60 frames a second, some players are able to push through it and get accustomed to the lower frame rate. After all, we watch movies at a frame rate, sometimes even lower than 30 frames per second, so it's not unreasonable to think that a game at 30 frames per second would still be enjoyable.
Suboptimal frame rates are much more important when it comes to VR. Since the HMD takes over the entirety of what our eyes see, it needs to update the world virtually as quickly as our eyes could. If the world we're perceiving doesn't update as fast as we look around it, our brain starts to get confused again and cue the nausea. This is sometimes referred to as VR sickness, and not only does it decrease the feeling of immersion the player gets, but it can also leave them feeling ill even after removing the headset.
With the hardware in the Oculus Rift, we can update the display with a new frame up to 90 times in a second. While 120 times a second would be even more ideal, a steady frame rate of 90 will be adequate in mitigating the vast majority of nausea.
Complex, asset-dense games that require a large number of calculations every frame can start to have a monopolistic impact on a computer's hardware, meaning your VR experience may start to drop frames. This should be avoided at all costs, because sporadic frame choppiness, known as judder, is one of the fastest ways to induce VR sickness.
