Windows instance takes ~18 minutes to restart

Hi all, my Windows Server 2019 m5d.2xlarge instance previously restarted quickly (~4min) but since mid-Sept 2021 it is taking exactly 18 minutes to restart. When I do "ping -t [IP]" the server is completely absent during that time, as if it were rebooting & chkdsk'ing just like old times.

I created a replacement fresh instance in mid-Sept 2021, Windows Server 2022 m5d.4xlarge based on the AWS Windows base AMI, and it also took a very long time to reboot nearly 30 minutes. I am now running my production workload on this instance so I have not rebooted it, but I'm concerned that it takes so long. My Win2019 instance is still around as a backup & I'm experimenting with it to make reboots normal again -- any ideas?

I have tried:

• stopped all non-essential services and set to Manual
• reviewed the eventvwr logs (it shows silence for ~18 min between typical last-event and normal-startup-event)
• run "fsutil dirty query c:" but it's clean
• ran "Sysprep /generalize /oobe /mode:vm" (which failed with unspecified error, but there was no change in behavior)
• did a "hard reboot" from Ec2 console (made reboot take longer, and Windows eventvwr complained about it later)
• I have already installed all Win updates

Any ideas would be appreciated, thank you!