I have noticed that the validatetrackback plugin was doing "expensive" work when it wasn't really necessary to do this. I have seen some trackbacks come from machines that are listed on real time dns blacklists.
Doing a dns check is a lot faster than downloading a html file and parsing through it, or even running the Bayesian filter. So I added an option in the validatetrackbacks script to check the black lists before doing any expensive operations.
I have posted a copy of the plugin here, but my hope is to check it into the tree in the next few days.