BuildTracer is the wondrous plugin that shows how long each tag takes to publish in an individual template. You may recognize it from its bands of colors. Credit goes to: http://mt.aklaswad.com/plugins/buildtracer.html .
I'm committing both v0.4 and v0.5. 0.5 definitely works with MT 4.3X, and I think 0.4 works with MT 4.2X. There was a difference in JSON code between some versions of MT, but I'm not quite sure when it happened.
When installed, go to Manage -> File Info, and click on an index template. You should see timestamps next to each template block to indicate how long they take to evaluate/publish, as well as an overall time for the entire template.
Install both parts of the plugin -- the plugin/BuildTracer code and the mt-static/plugins/BuildTracer code -- in order for this to work. You'll know if the mt-static code is missing if you don't see the bands of color.
Check out the build_timer.pl script if you want to benchmark all template times in a blog, since this plugin only drills down into one template at a time.