The container is slow to start, why?

If your $(id -u) or $(id -g) is not equal to 1000 then this would cause the zwift container to re-map all files (chown, chgrp) within the container so there is no uid/gid conflicts.

So if speed is a concern of yours, consider changing your user to match the containers uid and gid using usermod or contribute a better solution for handling uid/gid remapping in containers.