In this case it‘s about the actual startup time of your shell. When launching a new terminal, starship always need to perform its initialization. If it were slow, I wouldn’t use it because waiting seconds before being able to input anything is kind of annoying. That‘s what they’re referring to.
Great to see this, thanks for sharing! Do you have plans to make it available in nixpkgs and potentially as a home-manager or nix-darwin module? Then I could add it to my config and have everything set up automatically.