Downscaling "high" does make spritesheets quite a bit larger, but that also means images can end up in a different arrangement across spritesheets. That could actually result in fewer spritesheets being loaded in some cases, basically by luck.
Downscaling "low" does not make any effort to improve the quality of downscaled images, so if you see a poor result when downscaling an image, that shouldn't be surprising.