The trick with pinpointing a cause is that when I look at the data NR provides, every partial rendered and every action called is slower than it had been previously. Combined with the immensity of our codebase, it's going to be time-consuming to put a finger on a single repro-able cause. But we'll do our best to isolate and report back.
FWIW, have you guys ever considered creating a performance suite test app? With such sparse Google results around Rails performance changes over time, it could be useful to compare the performance of Rails versions on an apples-to-apples basis if we could see how the performance of the test app changed over Rails versions. If we're lucky, it might even catch some of the performance regressions that have happened in the dot releases previously. Even if not, it would at least demonstrate that performance is an important consideration in Rails' evolution. I'd sleep better knowing that.
Maybe take a popular mid-sized framework like Spree and see how it performs on 3.0, 3.1, 3.2, etc.?