Nice observation! That sleep is there in the case the queue is full, which means the caller can't put an additional element into it. The only option is to wait and see if the queue becomes less full... The hard question is for how long? Here we just added a fixed amount which was/is reasonable for our use-cases. Maybe/probably a better would be to implement exponential backoff https://en.wikipedia.org/wiki/Exponential_backoff, but that's a bit more complex (not too much). We never hit any limitation with the current implementation, if we do, we'll improve further!