User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

You just install Thunderbird 78.2.2 production version , not any betas.
It means do nothing.
I use old versions until now as I do not touch if it works correctly.
Version 17.0.9.5002 Delay on launch is zero (0) seconds.
Version 45.8.0.6273 Delay on launch is zero (0) seconds.
Version 78.2.2 Delay is 5 seconds, no matter default settings, is there accounts or clean after installation software without any, is format defined as mbox or maildir, no matter.
Windows the same , the hardware is the same, another instance of the Thunderbird mentioned above run no delay on the same profiles directory.

Actual results:

Always noticeable delay in 5 seconds.

Expected results:

Immediate open window of the Thunderbird program.

Your issue is likely one of these https://mzl.la/3mr30oH
You can follow the action in bug 487832