Yoga is like sex: When you're focusing on doing it right, you're doing it wrong.
Only ever matching the rhythm of your instructor, just hurrying along to get it done like the others, no time to listen to your body — that's not yoga.
When doing yoga, you should do it for yourself. Matching movement to your body is utterly personal. Other people can show you what to do, but you have to be in the right mental state to experience it. What is possible there makes you forget all the struggle, and you will be sure not to waste time on yoga.