You are just counting literally the time the first person starts talking till the last person ends, and even then its going to be closer to 10 minutes.

If you add up the time people have stopped working while waiting for the meeting, the time you take to gather people to a room, wait for the last one to arrive, start, go back, it's been 20 minutes total, and when standups derail and people start talking a lot it goes over half an hour.

You need to count the total overhead time. I mean come on, 10 minutes before the standup you are not really working full steam, right? You know you are about to have a meeting, so you are chatting with colleagues and checking email instead of working on your main tasks. That time should count too.