How often is ActionResponse.body a String? It's not really clear to me without digging deeper, but I could see this easily being an edge case related to certain caching and rendering methods. In any case, suggesting that any performance testing of Rails would have magically found this problem is unfair. Many many many Rails app instances are serving orders of magnitude more than 10reqs/sec, so clearly this is not a universal problem.

My guess as to the reasoning here is so that large files can be served without blocking while the whole thing is written to the socket. Granted, that's just my gut instinct looking at that code, maybe it is just careless...