I don't have access to a 3.x.x site, but
here is John Mitchell's recommendation for 4.x sites. You can probably use the corresponding settings and ignore the ones that you don't have in your site.
I think that 3.x can also take advantage of compression.
DotNetNuke Performance and Http Compression might be helpful. And, the aforementioned John Mitchell has a "PageBlaster" product at snapsis.com that might be an even easier way to implement compression. I believe that there is a free version.
Finally, it might be worth your while to upgrade to a 4.x version of DotNetNuke.
How to Upgrade DotNetNuke v3.x to v4.4.1 can help you to do this. If you do upgrade, you'll be upgrading to 4.5.3. I haven't done any actual speed testing, but the 4.5.x versions of DotNetNuke seem to have some zippiness missing in other 4.4.x versions.
Good luck!