Re: different execution time for the same query (and same DB status)

[Michael Lewis]

You don't mention shared_buffers, which is quite low by default. Not sure
of the memory of your docker container, but it might be prudent to increase
shared_buffers to keep as much data as possible in memory rather than
needing to read from disk by a second run. To test the possibility Tom Lane
suggested, do a manual analyze after data insert to see if it is also slow.
Explain (analyze, buffers) select... and then using
https://explain.depesz.com/ or https://tatiyants.com/pev/#/plans/new would
be a good option to have some visualization on where the query is going off
the rails.