Could it be made even faster using some of the ideas from https://github.com/zerobootdev/zeroboot ?

Their idea is actually more akin to cloning and a more refined implementation is used in Lambda's snapstart: https://docs.aws.amazon.com/lambda/latest/dg/snapstart.html

So directionally yes. But it relies on kvm. I focus on portability i.e. cross platform for local macOS and linux, so that is not top of mind for me right now.