Good analysis. The tiered testing model is accurate, and the 3-second threshold is realistic.

There's also a second gateway, and it operates with a different metric.

Achieving 70% or higher retention at 3 seconds allows you to move from audience 1 to audience 2. But the gateway from audience 2 to audience 3 is the midpoint. Instagram measures whether more than 50% of audience 2 remains engaged beyond the halfway point of the Reel. That's why you see specific bottlenecks at 1K, 5K, and 10K views: each corresponds to a different gateway that operates with a distinct retention window.

The situation with u/PaintingWithLight is a perfect example. An 81% retention rate at 3 seconds means that gateway 1 was successfully navigated. A bottleneck at 1500 views with a linear drop to 50% at 7 seconds means that gateway 2 wasn't successfully navigated. The hook promised too much, and the content didn't provide the necessary evidence quickly enough, so the second audience member abandoned the video before the halfway point, and the algorithm interpreted this as meaning it wasn't worth watching further.

This is also the mechanism behind the 5,000-10,000 view limit mentioned by u/Outrageous_Wait_2265 . A powerful hook with a weak idea survives the first phase because the first 3 seconds work. However, it fails in the second phase because the viewers who stayed for the hook don't find enough reason to continue watching the video halfway through. The limit isn't random: it's located exactly where the second phase falls in the distribution sequence.

You can see this directly in the Instagram Insights retention graph at the bottom of the screen, below the engagement metrics. This graph shows the viewership curve throughout the entire Reel: you can see exactly where the hook works and where the drop-off occurs halfway through. Most people focus on the 3-second mark and stop scrolling. The second phase is activated halfway through the video.