Primary-key choice vs database file size, H2 file database (MVStore B-tree), file size after the database is closed, 200000 rows per variant, batch_size=50, flush+clear every 50 rows
H2 specific: this is H2's storage engine, not PostgreSQL's or InnoDB's. Timings are indicative (one run, one machine).

variant                                   load ms     file KiB
bigint sequence (allocationSize 50)          1244         4944
UUID v4  random                              2280       101428
UUID v7  time-ordered                        1314        12960
UUID v7  as CHAR(36)                         1528        52948