Loading the latest N rows into the cache seems way too fast.

PG 9.6.24 and PG 14.15, if it matters.
(Yes, 9.6 is really EOL. I don't control that.)

(I could use pg_prewarm, but the table is much bigger than RAM, and
last_block value only has the newest record if data has never been
deleted. The oldest records regularly get deleted, and then the table is
vacuumed; thus, new records can be anywhere in the table.)

Thus, roll my own cache-loading statement.

The bigint "id" column in "mytbl" is populated from a sequence, and so is
monotonically increasing: the newest records will have the biggest id
values.
The table also has a bytea column that averages about 100KB.

Loading 200K rows is more than 200MB. I expected this "prewarm" statement
to take much longer than 1/2 second. Am I still in the dark ages of
computer speed, or is this statement not doing what I hope it's doing?

$ time psql -h foo bar -Xc "DO \$\$ BEGIN PERFORM * FROM mytbl ORDER BY id
DESC LIMIT 200000 ; END \$\$;"
DO

real 0m0.457s
user 0m0.005s
sys 0m0.004s

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