Slowness after 13.7 -> 14.4

Upgraded 13.7 to 14.4 on RDS by restoring from a snapshot and upgrading
that.

My configuration files and instance type are exactly the same. After
upgrading, I ran vacuumdb -Z -j 8 and that did its thing.

The upgraded database is slower. I'm running EXPLAIN ANALYZE for various
queries and it's getting to the same plan, but it's taking longer to
execute.

I've always felt after major upgrades there's some element to "priming the
database" where it seems to be much faster a few days in after broader
usage. I realize it sounds absurd but I'll swear by this being my
experience. So I'm willing to think to some degree that queries will
execute faster the more people use the upgraded database.

Just wondering if there is anything I might specifically look at that would
account for some of the slowness.

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com <wellsoliver(at)gmail(dot)com>