Bitdefender: Please try installing Adobe Creative Cloud along with Bitdefender on a Mac, then restart a few times.
You'll see files created at /Library/Logs/DiagnosticReports/shutdown_stall* whenever macOS is slow to restart. These are spindump files which contain hierarchical profiling of what was consuming resources. It's pretty much always Bitdefender and Adobe Creative Cloud, and I get one every time I restart. I don't see the lengthy stalls if I install either Bitdefender or Creative Cloud alone.