When using pre-generated GI cache files, they need to be blended when the final images are rendered. The blending can increase render times for the final images. It might end up cancelling out any time you save in the GI generation pass by only rendering 1/10 of the caches. On the other hand, if you're reusing GI cache files for multiple re-renders, you could save some time that way. They can be used on any image resolution, not just the resolution they were originally rendered at.