Some questions to dig deeper.
How many other users are in your Instant Pool and do not have the 10 minute issue?
Are you calling flexengine with GPO method?
Are you deploying the Instant Clone machines in the OU path to get the GPO.?
Is your base image configured to connect App Volumes manager? If it is, the base image machine name should show up in the App Volumes manager activity logs. That is usually a good sign that the connection to AppVol manager is working. Then once the pool is deployed, you should see the instant clone machines also show up in AppVol manager activity logs.
Hope these questions help you dig a bit deeper.
the active logs should be in your App Volumes Manager site. something like https://fqdnofmanager/activity#/Activity_Log
Clearly an issue with App Volumes. Please check AV logs.
It could be related to Hugh user profile loading during the logon. Let us know how the case with Vmware going