We're looking into moving to unicorn as the community has suggested. If anyone is interested in setting up Puma or Goliath, we'd be interested in testing those out as well.
It's worth noting that Rails' default JSON solution is the compatible-everywhere pure-Ruby JSON generator. Using a drop-in replacement like the oj gem will drastically improve throughput there.
I didn't get to the pull request for this round, but tweaking the GC parameters for the Ruby install should dramatically improve the general performance of the non-JRuby Ruby tests, as well. I'll see if I can get a PR in. :)