anybody have slow transaction/commits running under Xen?

I'm getting very slow transaction/commits with RoR 2.3.5 and Ruby
1.8.6 p388 running under Xen. The same virtual machine setup under
VMware runs the transactions fine. It's Ubuntu 9.10 64bit server.

Under Xen, it's taking 22ms to commit, according to the development
log. Under VMware it's taking 1ms or less. Doing it directly in
mysql console, the times are about the same, so I think it's a RoR
issue. The hardware is different, but the Xen hardware is much faster.

Anybody else encounter this? Any solutions? I've tried different
mysql versions and different Ruby versions/patch-levels.