There should not be such a difference between the two servers, sicne they share a lot in common. So I looked into the code yesterday, found the bottleneck and fixed them, as well as adding some tweaks. Here's what I did, and the chart[1] shows how the performance changed.
If you're running your site with Mason and want to port it into PSGI/Plack stack, you can help rjbs to complete HTML::Mason::PSGIHandler which I originally wrote in September 2009 for the demo purpose but never managed to finish (since I've never used Mason for any real project).