I found a bit of a glitch/technique with the character select in missions and thought I'd post this here. I'm pretty sure it's a bit faster than any other way of beginning a mission and I made a video showcasing it:

For those who don't want to watch the video or hear my voice (or if I didn't link the video properly): This works with all missions (Scout and Battle). To do it, you have to select the controls menu and, while still holding A, slide the cursor directly to OK. If done correctly and with the right timing, the game will transition directly to the Mission loading screen.