You still have to do a handshake (determine resolution, refresh rate, etc) before you get a picture, and that takes maybe 3 seconds. Some monitors can handshake a bit faster, but not by much.
- https://askubuntu.com/questions/1042641/how-to-set-custom-lo...
- https://support.apple.com/guide/mac-help/set-sleep-and-wake-...
etc