Can't make it any faster, well, I got it from like 5 minutes down to 1:30 on my p4, but, there are 2 alternative options / solutions:
Accumulative-add:
Let's say you add 1-2 new roms to fbn, go to the game list -> check show unavailable, find the roms you want to add and click the "[game info]" button & hit the "[rescan romset]" button - then go back to the gamelist and uncheck show unavailable.
My favorite:
No database mode (aka you'll never have to scan roms again, ever), It was originally an option for developers, but, I think anyone can benefit from it.
"Misc->Options->Scan ROMs at start-up" - Uncheck this. You'll probably have to quit & delete fbn.roms.dat from the config directory before loading. (it's been a while since I did this, and never looked back..)
best regards,
- dink