_refresh taking 30+ seconds

Is it normal for a POST _refresh on a single index to take longer than 30 seconds? That's what I see happening consistently on my 5 node ES cluster (running 5.0.0-rc1).

My application needs to call refresh because it needs to be able to search on documents that it has just inserted. Since the ES refresh rate is supposed to be 1 second. Would it be safe for me to just have my app sleep for a few seconds (rather than waiting 30 seconds for the POST _refresh to come back).

It also only seems to happen on one particular index. The other two indexes _refresh very quickly. And the slow index also only has a single document in it. Plus, I just discovered that index, update, and delete operations using refresh=true take a fraction of second to complete. So I guess that's a workable work around, but I'd still like to understand why it would take so long to do POST _refresh on that index.