You can do a check: Zip a bin (see how much time it cost) and then “unzip file.zip” and count again the time.
This time is the time that you will be waiting.
But if you tipically only play to one game, you can do some approaches: DraStic emulator (Nintendo DS Emulator for Android) have the option to keep a uncrompressed cache, that is, they uncompress the game that you play an keep it uncompressed: if you play again it don’t need to uncompress again, if you play other game it removes the previous game an uncompress the new. So if you play 2 times the same game the second doesn’t need the uncompression.
But Nintendo DS are smaller (< 100 MB) than PSX images (easy > 250 MB)