Slow disk performance on FreeBSD 5.3 beta

I'm benchmarking the latest beta of FreeBSD 5.3 against 4.10,
and am finding that disk performance is siginificantly slower.
(I've scanned the list archives and have seen that others have
reported similar problems.) Yes, I've turned off every debugging
option I can find, so it's not that. I have to build a high 
performance system for disk-intensive tasks this week, and so 
will use either 4.10 or Dragonfly (a fork of 4.x-STABLE) for it.

Has anyone identified the bottleneck? Is it in the ATA drivers 
themselves, or in another subsystem like geom or UFS2?

--Brett Glass