About 20x but we're not quite there yet.

https://youtu.be/WtAPUsGld4o

I did a very primitive version of this for rendering shadows for my master thesis. It sucked because of my poor accuracy eye tracker (self built) and because my approach was very naive.

Participants perceived jumps in shadow quality much harsher than lower resolution shadows.

It saved a lot of memory though, that was nice.

Microsoft has a great paper on more general foveated rendering.

That is an amazing concept - hopefully this can soon become reality