I've been testing an mp3 encoder for 68040 macs with fpu and here are some times I'm getting using Basilisk (Basilisk speed roughly equivalent to a 68040 25Mhz with fpu).
With a fast option (less quality) I get about a minute of an aiff file encoded in an hour at 128 kbps.
With the fast option off (higher quality) I get a minute of an aiff file encoded in 1 1/2 hours at 128 kbps.
If I encode at 192 kbps it doesn't seem to be that much slower.
If you have a 68040 with fpu idle you could use it to encode mp3s.
If anyone wants more details email me.