Very slow: ActiveRecord::QueryCache#call

I have an app on heroku, running on Puma:

workers 2
threads_count 3
pool 5

It looks like some requests get stuck in the middleware, and it makes the app very slow (VERY!). I have seen other people threads about this problem but no solution so far.

Please let me know if you have any hint.

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I will answer my own question: I simply had to check all queries to my DB. One of them was taking a VERY long time, and even if it was not requested often, it would slow down the whole server for quite some time afterwards(even after the process was done, there was a sort of "traffic jam" on the server). Solution: Check all the queries to your database, fix the slowest ones (it might simply mean breaking it down in few steps, it might mean make it run at night when there is no traffic, etc...). Once this queries are fixed, everything should go back to normal.