I was doing some testing on the data loading part of our pipeline and have been noticing a lot of problems when multiple concurrent insertMany operations are being triggered on a collection that contains a unique index.  For some reason MongoDB will just stop in the middle of the operations and the socket will stall, Mongodb will be using one or two cores at 100% CPU but there will be nothing in the logs, mongotop, currentOps or mongostat to show that anything is happening.  Eventually the connections will resume and finish the inserts without any errors but this may take anything from 2-10 minutes on my computer (my colleagues have tested it on their slower macbooks and have had stalling for up to 40 minutes).  In case it helps I don't believe the collection is locked, I can start another insert on another terminal with 1 connection and it will complete an insert of 1million rows without issue even when the original sockets are still stuck.

I have tested using docker to install a few different versions and this problem does not happen on v4.2, but does on v4.4 and v5.0.  On version v4.2 the test program will complete in 1.7 seconds where on the last v5 run I did it took 9m19sec.

I don't know if I'm doing something wrong, I should tweak some server configuration or if this is expected but it seems extreme.