In the HELP ticket repro, artificial fault injection was made to simulate disk error on mongod. While mongod was not capable to respect the operation timeout because the thread was blocked indefinitely on disk operation. Additional fault injection was made to simulate the operation timeout at mongos and that resulted in much slower connection buildup than without timing out the operations.

It should be assumed that interrupting a thread stuck waiting on socket reply should me much easier than interrupting the thread stuck on faulty disk I/O, because the TCP connection is still perfectly healthy. In Enterprise binaries, the problem of faulty disk on mongos is solved by Watchdog.