It's perhaps better to look at other examples such as Twitter [1] and Foursquare. The Yammer issues largely focused on extreme edge cases for performance, which have been improved in 2.10. It's not hard to write Scala code which runs at exactly the same speed as Java code, but is more expressive.

[1] http://www.wired.com/wiredenterprise/2013/09/the-second-comi...