On my system and a couple of others, Avast has the most impact on system shutdown speed. It takes the longest for the PC to shutdown with Avast than any other AV products.
I have "Verbose or Highly Detailed Status Messages" enabled on Windows, which tries to shows what services are being shutdown during the shutdown process.
There are a couple of ways to enable it. You can try this:
"reg add "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System" /v "VerboseStatus" /t REG_DWORD /d "1" /f"
For other AV products that I have tried, it doesn't show any AV related services being shutting, but for Avast Free it shows two like the images below. Avast spends the most time on the second one, "aswbIDSAgent".
If I remember correctly, a firewall related service is also shown if the Firewall is installed.
Avast should try to improve this.
Windows 11 Pro latest, Avast Free latest, No other security software is installed.