After updating from 2021 to 2023/6, I've encountered a huge performance issue with the package manager. It takes forever to process list of all packages in the project. It works fine in an empty project though.
After some debugging, the likely reason is the huge amount of GIT packages we got in our projects (around 150) - we got our own package managed that installs packages from our internal git.
Here is the reference to the forum topic on this issue:
https://forum.unity.com/threads/client-list-takes-forever-to-complete-after-updating-to-unity-2023-2-20f1.1605012/
I've attached our manifest, as well as logs from this project (upm_bigProject), and from a mostly empty project (upm_smallProject).
Note: No tests were carried out by CQA.