[Uberphallus]

That was my first thought. Especially when the article says

> Starting with Postgres 10, the default is to give 2 cores to parallel processing. Switching max_parallel_workers_per_gather between 2 and 0 had nearly zero impact on that Pi, either for or against.

With that extra info I'm 99% sure it's I/O limited.

[dleslie]

It's almost certainly IO. IIRC, Sdcard write speed on the RPi is known to be slow. You can improve it with superior cards, but the USB interface generally remains the path to fastest disk reads and writes.