Upgrade from 7.9.1 to 8.9.2 slowdown on postgres queries

We just upgraded from Version 7.9.1 (build 27448) to Version 8.9.2 (build 46101), using the same PostgreSQL version as before 11.4.
The steps were done as in the docs: upgrade, modify configs, migrate db(~6h) and vacuum db.
The issue we encounter are:

  • Slowdowns on report processing(background tasks) from few seconds to couple of hours.
  • Slowdown on web display for issues from few seconds to ~1 min.
  • Web display for Projects returns 0 projects after ~1 min with timeout error in browser console and no error in logs server side, just the query in db that is still active.

Re-indexing the db did not solve the issue.
Can you please advise to what we can do in order to resolve this or if we any of the steps are done wrong in the process of upgrading?

Thank you!