Performance Issues on AMI deployed Build Server

So I made a Build Server (Windows Server 2016 on a T2.Small) with everything needed, and had it working directly in TeamCity without issue. Builds were 3-4 minutes including deployment. 

I then connected that EC2 instance via the Cloud Integration, and it also worked nicely. Starting and stopping the single instance without issue. I couldn't work out how to share that one EC2 instance in Cloud Integration across many projects however, so moved to an AMI.

I took an AMI from that instance, and have wired it up into the Cloud Integration. Everything *works*, it's actually really cool. I've even gone as far as enabling spot pricing, etc. very cool. 

but its slow. A build now takes 10-15 minutes where it is much quicker on the non AMI image. I can understand it taking longer to come online, deploying a server form the AMI obviously takes longer than just powering on an existing server, that's fine, but this is after its booted. Most of the speed issues seem to be in the GIT stages, the build logs show that "compute revisions" takes a LONG time. the rest isn't so bad.

I even tried bumping up to c5 and m5 instance types to see if that helps, and while it adds some speed to the actual build steps, it doesn't help this initial GIT step. 

Any ideas why the AMI version of the Build Agent would be so much slower? (I even tried re-activating the EC2 instance I took the AMI image from, and it's as fast as always, despite being a t2.small!)