Notes about Pl/PgSQL assignment performance

Few day ago a faced a problem: Pl/PgSQL procedure works slower when running
in parallel threads. I found the correlation between number of assignments
in procedure code and performance. I decided to write the simple benchmark
procedures and perform some test on PostgreSQL 9.6.5 database installed on
the server with 20 CPU cores (2 Xeon E5-2690V2 CPUs).

This benchmark showed me that a simple Pl/PgSQL procedure with a simple
loop inside works slower when running even in 2 threads. There is a
procedure:

CREATE OR REPLACE FUNCTION benchmark_test() RETURNS VOID AS $$
DECLARE
v INTEGER; i INTEGER;
BEGIN
for i in 1..1000 loop
v := 1;
end loop;
END;
$$ LANGUAGE plpgsql;

What is the point? I know, that Pl/PgSQL performs a SELECT query to
calculate each value for assignment but I didn't expect that it produce
side effects like this. If there is some buffer lock or anything else?

I've been written a post with charts and detailed explanation to display
these side effects:
http://telegra.ph/Notes-about-PlPgSQL-assignment-performance-12-19

Any help would be greatly appreciated.
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