Does modifying parallelism improve search throughput?

[zabeen]

In prod, we regularly see the search queue becoming "blocked" by long-running mismatch searches.

We have various dev tickets around this: see #1282

However, we could potentially ameliorate the issue by playing with existing search config settings, as documented here, here, and in this ticket.

[zabeen]

Awaiting test plan - to be constructed in collaboration with @RimkePostel