I ran some timing tests, partly to check out -fast-math -- my informal impression is that it speeds up exp by a factor of about 2, but can slow down other things (FIR filters). I didn't see any obvious improvement in -O3 over -O2. The actual numbers are in the snd tarball, snd-edits.c -- look for "fast-math". (The table is aimed more at comparing int/float/double operations). These timing numbers change by a factor of, say, 20% on each run, so don't put much weight on them.