Network/Load-balancer programming is embarrassingly slow on Flex. We have long term projects in place that will improve it but I can't give any dates here.

Short term we have a few tactical features in the pipeline. In the next few weeks we are rolling our "parallel build and deploy" which moved the docker build to run parallel with LB programming. Depending on your build that saves a few minutes.

When doing development I usually just replace an existing version by deploying with:

gcloud app deploy --version <my-dev-version>

This keeps the same LB and VMs as before and does a gradual container swap. It is not safe for production but definitely helps when iterating.

This is exactly the reason why we went away from appengine flex to AWS Beanstalk. As much as a google fan I am, this is unacceptably high and there seem to be no interest by Google to reduce this. Beanstalk isn't that great either but better than appengine flex when it comes to the deployment wait times.