I’ve ran Boston twice. First time I ran it 19 minutes slower than qualifying. That was because I didn’t train as hard and was treating it like a big vacation. We did every tourist sightseeing thing you could do in Boston. I had 25k steps per day for the two days leading up to the race.
Second time, I trained harder than ever. My predicted time based on my tune up half marathon in March was 2:43 high. So I paced Boston for a 2:46 high since it’s a tough course. Was on pace for 2:46 through mile 24, but ended up bonking with about 2.5 miles to go and shuffled in two minutes slower than my qualifying time. I think the bonk can be partially attributed to the late start in Boston. I never wake up at 5:30am to run at 10am so I don’t think I ate enough. Plus, if you’re catching the buses downtown, you’re walking nearly 10k steps before the race to get to gear check, catch the bus, walk through athlete village, get to the starting corrals which are almost a mile from athlete village, etc. That walking didn’t bother my legs, but it’s a lot of extra calories burnt before starting a difficult course.
I think both of my scenarios are some of the common reasons people go slower at Boston.