v3.7.1 reindexing much slower than v3.7.0

Created by: slimsag

It appears that with the new v15 shard reading support reindexing after upgrading to v3.7 is much slower on the same exact set of repositories. It doesn't appear to be a resource constraint of any sort, and most of the time zoekt-indexserver is sitting idle. I suspect some sort of indexer scheduling issue?

In specific, I am certain with the same set of repositories that:

• upgrading v3.5.2 -> v3.7.0 (zoekt: 014a202) reindexing completes at a little under 6,000 repos/hr
• upgrading v3.6.2 -> v3.7.1 (zoekt: 4e4eef5) reindexing completes at just 1433 repos/hr

It's not the end of the world (we can always advise users to just wait longer), but I would like to understand why it is so much slower on the same set of repositories and see if there is a simple solution.

@keegancsmith Any idea how the following changes could cause this? https://github.com/sourcegraph/zoekt/compare/sourcegraph:014a202...sourcegraph:4e4eef5