Just disable or uninstall bitdefender for the time being. You can always add it back.
At first, I was reluctant to permanently uninstall Bitdefender for troubleshooting because it might be hard to reinstall it and have it recognized as a paid for subscription.
But it came to the point and you are 100% correct. So I uninstalled BD and did 3 shutdown and power on boots. All 3 boots into Win11 display the desktop icons instantly when the taskbar is displayed.
So yes, it definitely proves that BD was the culprit that was delaying the desktop icon display for 51 seconds. (Unless tomorrow morning's power on boot has the delay problem again 😉)
The delay does not happen on my Win10 tower PC or my Win10 laptop PC. So it appears that BD is not fully compatible with Windows 11.
I've sent an email request for help to the Bitdefender company and received a confirmation number. Will see if Bitdefender has a fix.