Unless all your tables have the same width - or you’re doing weird things with constants in your SELECTs - you can’t UNION the various queries, so they’re sequential. You could parallelize those I suppose, but now you’re adding more complexity.

If you want a KV store, use a KV store. If you want an RDBMS, then use its features. They haven’t changed much in the last 50 years for a reason.