I tested it. It's about 3 seconds when switching to my Mac Mini (where this is the one and only monitor) -- it takes a couple of seconds longer to switch back t

I tested it. It's about 3 seconds when switching to my Mac Mini (where this is the one and only monitor) -- it takes a couple of seconds longer to switch back to my laptop because I use the laptop open and macOS seems to take forever to think about which windows to move around randomly before lighting up the external display.