Have you seen it be faster in the past with the delete-by-query plugin?
The delete-by-query plugin and they reindex API coming in 2.3/5.0 are single threaded things built on top of the scroll API. They aren't designed for speed so much as simplicity and stability.
You are generally better figuring some way to delete whole indexes at a time rather than by query. That being said, you can parallelize the process by launching the API more than once with constraints on it that make sure it doesn't overlap. Like "delete all documents with this tag" and "delete all documents with this other tag", etc. If they overlap then they are unlikely to get any speed, though they will probably still work.