One of the problems of immersive VR is that you need very high precision near the fovea ( = at the point of screen you are looking at), and you need to render very fast at the edges of vision. This means that you need to render at a very high resolution and at a very high frame rate. This is, of course, a problem.

However, while the vision at the fovea is very sharp, it is also very slow, and while the vision at the edges is very fast, it is very blurry and imprecise. With eye tracking, this is exploitable. The practice is called foveated rendering, and the idea is to only draw full precision near the position of the screen you are actually looking at, and elsewhere render at a much reduced resolution but at a higher frame rate.

In tests, this has been found to be impossible to distinguish from just drawing the full screen at full resolution and speed.