Found test_benchmark, a true gem (pardon the pun) of a Rails plugin. Times each test case, then reports the top 10 slowest, or all if you ask e.g.
rake test:all BENCHMARK=full
You can also disable it by default if you add
ENV['BENCHMARK'] ||= 'none'
config/environments/test.rb, as per this post. Very nice.
One problem: init.rb tests for autotest, using not-so-Windows-friendly logic. This can easily be patched with a small if-not-Windows-then... guard around the offending 'ps'/autotest loop. See the logged issue for more details.