30 seconds works the same for everyone. Why was an old Brit faster than 30 young Chinese??
It wasn't 30 young people. Watch the video more carefully. The girl slips and falls. The lady filming the video calls out for help immediately. People start yelling. The camera angles down at this point, but you can still see that only 2-3 people standing on the bridge could see exactly what was going on. Right after that, the girl passes under the bridge, so a lot of people wouldn't even know what was going on. Then the elderly man takes off his shoes and jumps into the river at exactly the 36 second mark. Less than 10 seconds later, someone tosses in a buoy.
The bystanders only gathered after the man jumped in, at which point everything looked to be under control and there was no point in doing anything else.
Like I said, kudos for the elderly British gentleman for having the presence of mind and the courage to save the drowning girl, but you're being too harsh. People react differently to sudden events, some people just freeze up. The British gentleman who jumped in was a triathlete, very likely he also had some experience as a life guard or had some training, or simply had more presence of mind due to his athletic background. Whatever the reason, he was there, he acted quickly, and the girl was saved. There's no reason to be apportioning blame here.
I remember very clearly one incident when I was young. I was at a park in Shanghai with my parents, we were walking near a lake. Someone yelled out a kid had fallen in. My dad goes to see what's going on, and by then the kid had already climbed back on shore and was wringing dry his clothes. It's just one of those situations that happen very fast, and if you're not on top of it from the start, it will be over before you realize what happened.