we only use a single thread with sqlite

[richvdh]

For reasons that are unclear to me, we only use a single thread with sqlite by default (see here).

I feel like this may be responsible for some of the poor performance seen with sqlite.

[richvdh]

One reason that we're only using a single thread might be to do with the blocking behaviour without Write-Ahead Logging enabled (according to #2917 (comment), "Non-WAL mode only supports either N concurrent readers or 1 writer, so writes can indefinitely block reads").