COPY for distributed table slower than regular table in PG13

I created tpch's lineitem table in a 1 coordinator+2 worker node local cluster, all nodes in localhost. Postgres version was 13. I was doing this in WSL2 Ubuntu.

When the table is not distributed, COPY command takes 15s to finish. When the table is distributed it takes 27s.

It might be because of the COPY improvements in pg13 which we might have missed to used in Citus. COPY in pg13 seems to be faster than pg12. I don't remember the reason, but I think Andres knew the reason & confirmed it when I was working on columnar.