[PERFORM] WAL + SSD = slow inserts?

[Skarsol]

I'm trying to increase the speed of inserts in a database that is on a not
super fast storage system. I have installed a pair of SSDs and placed
pg_xlog on them but am still getting inserts that take up to a second to
complete, with .3 seconds being about average. Iostat doesn't show the SSDs
stressed at all, and changing synchronous_commit doesn't seem to affect it
one way or the other. Where would I look next for what could be causing the
delay?