The server default configuration started using UFC_NB and MFC_NB in order to avoid blocking on the Netty event loop threads, slowing client requests, and then the core default configurations started using UFC_NB and MFC_NB as well with ISPN-9886.

Unfortunately the _NB variants aren't really non-blocking: they have an extra queue for messages that require more credits, but that queue is limited (2MB by default), and once the queue is full, they also block. They also have much more complex locking than the blocking variants: we actually had to increase max_credits from 2MB to 3MB in order to get the same performance with UFC_NB that we had with UFC.

We're changing the defaults back to UFC and MFC while we investigate possible alternatives to avoid blocking.