Horribly Slow!
Earlier versions are super slow to launch (pass basic status checks), and super slow to bootstrap (respond even on ssh port). The latest version fails to launch fast enough to get past the basic status checks (10 minute timeout), and there is no means to alter this timeout for an auto scaling group (not the same timeout has HealthCheckGracePeriod). If you're designing for high availability, this is not the product for you.
There are means to alter the health check timeout by "initial_lifecycle_hook" and setting the "heartbeat_timeout " https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/autoscaling_group#initial_lifecycle_hook https://docs.aws.amazon.com/autoscaling/ec2/userguide/lifecycle-hooks-overview.html If you are having issues with the deployment of VM-series i could recommend deploying the pre-packaged terraform samples that palo alto has on their github https://github.com/PaloAltoNetworks/terraform-aws-vmseries-modules.