At the start of the loop doze off at the camp fire for around 10 minutes 45 seconds (10:45), then fly to the stranger and enter through the hangar. When you get inside you'll be able to see the dam on the left almost ready to break. Jump on the raft in front of you and start rafting around the stranger.
For the route around the stranger, I stayed to the left as much as possible and took the left most path each time there were multiple path options or the path split. The only spots I did not go left at was when I was passing through the broken dam, I took the right opening as it is much wider and easier to pass through than the opening on the left side. And also in River Lowlands after the dam breaks, I would get as close to the house on the right corner as possible until getting around the corner, as it seems much faster and more direct than going left around the buildings. I'm not sure if this is the fastest route but it does works.
About half way around the stranger, the dam should break which will cause the water to flow more and you to go faster in some areas. You can catch the wave if you timed your initial take off on the raft well but it's not needed for this (I was always too far ahead to catch the wave). Timing the dam to break half way around your first trip also gives you much less obstacles like other rafts and broken structures to deal with, and once you reach the dam area after it breaks, you should get a massive speed boost all the way from the start of the dam area and through River Lowlands (for some reason I never get the speed boost through this area unless the dam recently broke, but maybe I was doing something wrong?)
I'm also not sure if it's faster to hold your torch on two of the lights on the raft instead of one, but it does feel like it's faster. If your raft is angled in a way which allows you to do that, I definitely would recommend it.
This is probably over-complicating a simple achievement, but I either continuously lost my run to random floating objects or I just couldn't get anything better than around 95 seconds at best without the speed boost in the dam area, so I hope this can help anybody else also struggling with this.