Spinning class means inside bikes... I wouldn't worry about the speed at all. (BTW, I spent years going 10-12 mph.) Effort is 'way more important, as is technique. Its' not like you're going to get left behind!@!
When you can get outside on a "real" bike, then you can actually measure speed under actual conditions.
And... what *really* matters is all yours to decide anyway Fer cryin' out loud... is somebody going to walk intot hat spinning class and say YOU ARE NOT WORTHY... I don't think so Those hollywood agents are also not too likely to drop by and offer us a movie role, too... so it's up to us to do things for our own reasons...