2019.3.a11: Opening my scene takes forever

My project has just one scene, but I have substantial amount of data serialized into one scene object (legacy code). Scene size is 21 mb.

 

In 2019.3 they worked on serialization performance, see this thread:
https://forum.unity.com/threads/performance-improvements-to-text-serialization-in-2019-3.661993/

I guess it's possible you found a case they didn't test / optimize with the new system. If you're able to reproduce this issue, I believe it would be best to submit a bug-report, so they can take a look at the issue and test their fix against.

Please submit a bug-report as described in this document:
https://unity3d.com/unity/qa/bug-reporting

It's important that you report these issues together with a reproduction project if you want them to get fixed. If you don't do it, it might be a long time until someone else reports them or until Unity Technologies find them.

After you submitted the bug-report, you receive a confirmation email with a bug-report Case number. Please post the Case number (number only, not the link) in this forum thread for Unity staff to pick up.