Wanted to address this as we're seeing this topic pop up here and there. When we deployed the new launcher, we stated "The “download” in this case refers not to re-downloading the game client but downloading small pieces of information for validating the game data already on disk. So to reiterate once more, players will not be experiencing a full download."
That is still the case when switching between live and PTS. You are not experiencing full downloads, rather, you are seeing a sync with your internal files. Going back and forth between the servers will always have this syncing period with them.