Yes you should always copy your ROMS to FASTRAM, this will speed up everything you do in WB, not just ScummVM!
If you have a Blizzard you should look at Blizkick (which will also speed you your IDE because it has SpeedyIDE built in).
I've built a custom 3.9 ROM which I load into FASTRAM when I boot with my ACA so I don't even need a soft reset
If you're having stability issues you should check your system for patch conficts and also run a memory checker because my system is very stable with ScummVM AGA v1.3.x.
BTW v1.3.1.003 060 has been released to AmiNet, should be faster still