Very, very good question. The long-story short is that I don't think there's an algorithm that can determine the completion of page-load with certainty for comp

Very, very good question. The long-story short is that I don't think there's an algorithm that can determine the completion of page-load with certainty for complex JS SPAs. We fingerprint elements, check if the page is changing along the critical path and wait if it is, and look at network traffic. We can't say with certainty, but we can form a good guess - the accuracy of that guess is always improving. This is something I think would benefit from open-source collaboration, if we went down that route.