There are a number of compression algorithms that prioritize decompression speed, usually at the expense of higher compression times.

It can actually be quicker to store them compressed because memory and bus bandwidth is often a bottleneck. So instead of the cpu or gpu wasting cycles waiting for data to be moved, some of that movement time is shifted to the processors by using compression. Especially if there are idle cores that could be put on that task.

As for going from one compression format to another, you could store them in the final format (and convert on install if it differs between hardware setups, repeating if another hardware setup is detected).

Though if there's any processing done on the uncompressed data (like generating mipmaps or something), that conversion might not even cost extra because it needs to be decompressed and the new data compressed again anyways.

Though on that note, you'd get faster load times by just storing all of those preprocessed and faster install times by just sticking it all in the install download, so there is still a conflict between optimal load speeds and minimal storage space.