Yeah, the system requirements for DF are probably orders of magnitude higher than Crawl.
Opinions on what constitutes acceptable FPS for Dwarf Fortresss vary a lot depending upon how used to being spoiled with incredible processing power the players are, personally I started playing DF on a pretty old lap top a couple of versions ago, and was happy to have 40 FPS with a 3 x 3 embark, no river, 80 dwarves, rain and temp switched off. Now with the current DF2010, and my new laptop, I am happy at 40 FPS, a 4 x 4 embark, a river, 80 dwarves, rain and temp on.
If your laptop is really old, then you might want to play an
older version of DF, like 31.08, or even 40d, which should be compatible with the
nanofortress utility, which allows 1x1 embarks. Generate a small world, with a short (50 year) history, use a 1 x 1 embark, cap your pop at 50, kill all your cats, switch off rain and temp, be happy.
Whereas crawl, and particularly Putty, you can just play