I've tried a little experiment over the last few months. When I get one of those warnings telling me an SPMS scan is coming up, I grab the scanner before I get to that address, stop at that address and start counting "one thousand one, one thousand two, and so on." It takes me to 3 to 4 before the scanner goes off wanting me to do the scans. So I'm guessing 3 or 4 seconds is how long it takes for the GPS bread crumbs to register with the "system." In other words, your PM is likely correct.
BTW, the highway patrolman that invented that white line system was from the county I grew up in. Basically, if you're going 60mph, it should take you exactly 1 minute to travel a mile. For each second you're under a minute, that's 1 mph over 60mph you're traveling. For each second over, you're traveling 1mph slower than 60mph.