> Actually rendering at a lower resolution in areas you aren't actively looking is probably the most complicated approach because it seems like you'd have to render two times (low res + high res) to get the desired effect

In the newer implementations, you can split the display into tiles, and render each tile at a different resolution, so it's not that bad. A bunch of the intermediate rendering targets are already rendered at reduced resolution, so you won't see a full 4x speed up in the low-res regions, but you should be able to achieve a ~30% overall speed up with modern foveated rendering.