How do I quickly tell how serious an emergency call is?

This is the hardest part of the job and the part no one is good at right away. Reading a call fast, judging how serious it is in seconds, is a skill built from repetition, not something you either have or lack. That you cannot do it fast enough yet means you are new to it, not that you are wrong for it. Speed here comes from pattern, and pattern comes from volume. Every call you take teaches your mind what serious sounds like, until one day you recognize it before you can explain why. The experienced people around you are not smarter than you. They have simply heard it thousands of times. You are building that library right now, call by call. Use a simple frame to speed up while you learn: for each call, ask first about breathing, bleeding, and consciousness. Those three answers sort most calls quickly. Lean on that structure until your instinct catches up. It will, faster than you think.