Thanks for the effort.

Probably nitpicking but these types of measures are usually tricky to interpret because there is a high chance your indexes (maybe even rows) are still on PostgreSQL shared buffers and OS cache and might not reflect real usage performance.

To get a more "worst-case" measure, after your inserts and indexes creation, you can restart your database server + flush OS pages cache (e.g. drop_caches for Linux), then do the measure.

Sometimes the difference is huge, although I don't suspect it will be in this case.