Rails running on Matz's Ruby Interpreter (the de facto reference interpreter for Ruby) had been criticized for issues with scalability.  These critics often mentioned various Twitter outages in 2007 and 2008, which spurred Twitter's partial transition to Scala (which runs on the Java Virtual Machine ) for their queueing system and other middleware .   The user interface aspects of the site continued to run Ruby on Rails  until 2011 when it was replaced due to concerns over performance 
A little more drastic, we’ve also pushed all the commercial database adapters into their own gems. So Rails now only ships with adapters for MySQL, SQLite, and PostgreSQL. These are the databases that we have easy and willing access to test on. But that doesn’t mean the commercial databases are left out in the cold. Rather, they’ve now been set free to have an independent release schedule from the main Rails distribution. And that’s probably a good thing as the commercial databases tend to require a lot more exceptions and hoop jumping on a regular basis to work well.