I find the key to speed is living with mistakes and not letting the pursuit of perfection get too much in the way of things; we all want our art to look immaculate but the reality is, nothing is ever going to be perfect and you can't make art for a living if you can't finish your work in an efficient-time. Same thing for when I was in school. It was better to hand in good work at the deadline than not hand in anything at all because I was trying to get it to be perfect.
But I found the best things to do to improve drawing time is gather all your references before you start drawing and just keep practicing. The more you know anatomy and perspective the less hangups you get along the way.