I don't think that's how it works. Within a single dyno with concurrency of 4 a single request can not block block any requests. It can only block 1 thread, but

I don't think that's how it works. Within a single dyno with concurrency of 4 a single request can not block block any requests. It can only block 1 thread, but then the other requests will be processed on the other threads. Routing is only random to dynos, within dynos it is not random.