Similar problem for me. I'm using Win 7 Pro x64 (all updates) and Avast Premium Security 20.4.2410 (build 20.4.5312.577). No other antivirus or anti-malware software installed now or at any earlier date.
Whenever I reboot, the system hangs for a very long time (at the "shutting down" screen) and eventually reboots. Fortunately, there's no slow down or hanging when I shut down, although the Event ID warnings/errors listed below still occur.
Shutting down shields (all or any combination) has no effect whatsoever on the slow reboot and hang time. I tried this with both Self-Defense mode on and off, so that has no effect either. Both Silent Mode and Hardened Mode are switched off.
Interestingly enough, I found that if I switched the Program Compatibility Assistant Service (in services.msc) from "automatic" to "disabled," the error (Event ID 7043) didn't appear in Event Viewer; the warning (Event ID 1530) was still recorded, however. Reboot time wasn't decreased, so there was no point in keeping PCAS set to "disabled."
Here are the full results in Event Viewer:
Event ID 1530 (User Profile Service)
DETAIL - 3 user registry handles leaked from \Registry\User\S-1-5-21-4097084660-844498693-2369078883-1000:
Process 1364 (\Device\HarddiskVolume3\Program Files\AVAST Software\Avast\AvastSvc.exe) has opened key \REGISTRY\USER\S-1-5-21-4097084660-844498693-2369078883-1000
Process 1364 (\Device\HarddiskVolume3\Program Files\AVAST Software\Avast\AvastSvc.exe) has opened key \REGISTRY\USER\S-1-5-21-4097084660-844498693-2369078883-1000
Event ID 7043 (Service Control Manager)
The Avast Antivirus service did not shut down properly after receiving a preshutdown control.
Process 1364 (\Device\HarddiskVolume3\Program Files\AVAST Software\Avast\AvastSvc.exe) has opened key \REGISTRY\USER\S-1-5-21-4097084660-844498693-2369078883-1000\Software\Microsoft\Windows\CurrentVersion\Explorer