Table with large number of int columns, very slow COPY FROM

Hi,

I have a set of tables with fairly large number of columns, mostly int with
a few bigints and short char/varchar columns. I¹ve noticed that Postgres is
pretty slow at inserting data in such a table. I tried to tune every
possible setting: using unlogged tables, increased shared_buffers, etc; even
placed the db cluster on ramfs and turned fsync off. The results are pretty
much the same with the exception of using unlogged tables that improves
performance just a little bit.

I have made a minimally reproducible test case consisting of a table with
848 columns, inserting partial dataset of 100,000 rows with 240 columns. On
my dev VM the COPY FROM operation takes just shy of 3 seconds to complete,
which is entirely unexpected for such a small dataset.

Here¹s a tarball with test schema and data:
http://nohuhu.org/copy_perf.tar.bz2; it¹s 338k compressed but expands to
~50mb. Here¹s the result of profiling session with perf:
https://pastebin.com/pjv7JqxD

--
Regards,
Alex.