The duron is a pretty low spec processor (not much cache). You need at least a 1.4GHz Athlon to run UAE with the sound on 100%. It worked fine on my old Tbird.
If you remember onboard audio takes a good 10% of your CPU compared to a soundcard. It doesn't sound unreasonable that emulating an Amiga chipset takes a lot CPU power. I hope they get around to optimising that.
