Yes caching would be a good thing..
or
a faster way to list them would be to not get the mapper info etc. and just let the user pick what they want listed next to the game? It would shave off about one second for those people with huge rom collections.
Also, perhaps a subdirectory depth of 1 folder or X folders would be nice as I've got my roms in Alphabetical folders for storage, but would be nice to be able to browse them all.
Edit: just let the browser run through my entire set and it took a good 50seconds on a 2.4Ghz Pentium 4 and a ATA133 HDD.. but one thing, the rom listing kept going when i opened a ROM through the file browser, perhaps you should stop the thread there. (it didn't slow the emulator down or anything but the HDD was really thrashing about).