I think we have seen a similar issue. I have only paused when the engine has gone stalemate, and then it's waiting in SoundEngine::LoadBank on the main thread and in FPakPrecacher::QueueRequest on the thread IOSchedThread.
For us it was temporarily resolved by disabled the new "Event Driven Loader" that comes enabled with 4.15. You can find this in Settings > Project Settings > Streaming > Event Driven Loader Enabled and uncheck that checkbox
I hope this helps you in the wait for 2016.2.4