Thanks Matt! I really enjoyed your blogpost as well.
We didn’t pinpoint the exact causes but the suspicion was that the JSON serializer was negatively affected by the upgrade (either directly or because of other changes in Rails 4), and slowed down the presenter models as a consequence. The Web team was playing around with Jbuilder, and we were trying out MessagePack, but I don’t think we have thorough benchmarks for comparison.
The upgrade happened a couple months before my co-op term started, though, so my understanding of the situation may not be the full picture.