I went through the same mental process as you and also use num_cpus [0] but this is based only on intuition that is likely wrong. More benchmarking is needed as my benchmarks show that more parallelism only works to a point.
You can see how the transactions work in this example[1]. I have a connection `.write()` or `.read()` which decides which queue to use. I am in the process [2] of trying to do a PR against rusqlite to set the default transaction behavior as a result of this benchmarking so hopefully `write()` will default to IMMEDIATE and `read()` remains DEFERRED.
