They can open nearly instantly, but they need to be well-built native apps. Lumbering behemoths like Adobe CC or anything Chrome-based is going to be a bit slow, and even smaller apps that pull in a big foreign UI toolkit are going to take a tick or two longer to start.

It’s too bad that the art of building lightweight applications is becoming a lost art.

On the current M series hardware, in my experience, it's rare for the icon to land from its first bounce before the software is up and ready.