We ran into this exact same problem at Impact Dialing. When we hit scale, we optimized the crap out of our app; our New Relic stats looked insanely fast, but Twilio logs told us that we were taking over 15 seconds to respond to many of their callbacks. After spending a few weeks working with Heroku support (and paying for a dedicated support engineer), we moved to raw AWS and our performance problems disappeared. I want to love Heroku, but it doesn't scale for Rails apps.