Response times quadrupled and we would get many timeouts.
We use this buildpack in Heroku: https://github.com/HashNuke/heroku-buildpack-elixir
We did several ENV var changes, playing around with the DB poolsize, all to no avail. In the end we changed one line in the buildpack:
# Always rebuild from scratch on every deploy? always_rebuild=true
Setting this to true instead of false fixed our problems.
So this must means we had a rogue build or something. I must say that this sometimes (very rare) happens to my or my co-workers. Our phoenix app in dev mode will all of a sudden have terrible (seconds) response times. Cleaning the _build folder and recompiling then fixes everything. It seems a similar thing happened on heroku.