performance regression when filling in a table

Hello devs,

On my SSD Ubuntu laptop, with postgres-distributed binaries and unmodified
default settings using local connections:

## pg 11.2
> time pgbench -i -s 100
...
done in 31.51 s
# (drop tables 0.00 s, create tables 0.01 s, generate 21.30 s, vacuum 3.32 s, primary keys 6.88 s).
# real 0m31.524s

## pg 12devel (cd3e2746)
> time pgbench -i -s 100
# done in 38.68 s
# (drop tables 0.00 s, create tables 0.02 s, generate 29.70 s, vacuum 2.92 s, primary keys 6.04 s).
real 0m38.695s

That is an overall +20% regression, and about 40% on the generate phase
alone. This is not a fluke, repeating the procedure shows similar results.

Is it the same for other people out there, or is it only something related
to my setup?

What change could explain such a significant performance regression?

--
Fabien.