All of that might be true, but it doesn't answer the question: Why isn't the Migration Assistant time prediction algorithm more accurate? Part of this might be

All of that might be true, but it doesn't answer the question: Why isn't the Migration Assistant time prediction algorithm more accurate? Part of this might be the fact that the author is (or at least appears to be) using a traditional magnetic HDD, rather than an SSD. I say this because I've transferred many a MacBook Air over Thunderbolt in 40 minutes for smallish amounts of data. The longest time I've seen MA take was 4 hours for a MacBook Air with a 256 GB SSD (don't recall the utilization). A 10 hour transfer time smells of magnetic platters. The reason HDD vs SSD would impact prediction is that the throughput of an HDD is impacted more severely by a greater number of variables when transferring at the filesystem level than a SSD. Man, that was a mouthful. Stated simply, HDD performance is less predictable than SSD performance. Given the direction Apple is headed with primary storage, I could see them not investing much in tailoring their estimation algorithm toward a technology that is on the wane.