Ruby Threading Performance: Slow Thread.pass

We're running a threaded ruby server (Puma), and have seen serious performance issues with our Sinatra app. Specifically, something as simple as Thread.pass can take over 2s. How is it possible that a server with 16 threads can take over 2s to return control to a thread? Is the Ruby scheduler that bad, or is there something we can do to fix this?

Details:

• Ruby implementation: MRI 2.1
• Sinatra App
• Running on Heroku 1x dynos
• Puma server, running 16 threads, 1 process
• Some routes are doing fairly heavy work, but routes doing almost no work are impacted
• Over 100MB in free memory

Thanks in advance!