BUG #15008: Need a pause

The following bug has been logged on the website:

Bug reference:      15008
Logged by:          Laurent Martelli
Email address:      martellilaurent@gmail.com
PostgreSQL version: 9.6.6
Operating system:   Debian 9.3
Description:

I have a set of scripts I execute with psql, and for some reason, a query in
one of them take forever (never had the patience to wait for more than 15
minutes), eating 1 CPU, unless I insert a pause (sleep 60) before that
script, and then it only takes a few seconds.

It only occurs when the tables we operate on have a sufficient number of
rows. When we do "unit" tests with a subset of the data, it all goes
smoothly.

Please, tell me what additional information I can provide to help fix this.