Amazon has a thing about book 1's. They take about 3 times longer than any other book to go through. No idea why.
It's the book most likely to be price changed for promotional purposes. I'd guess there are a heck of a lot more requests for book 1 price changes. That could be why it takes longer.
If that was the case, then all books would be just as slow. What you're saying suggests book 1's have a completely different assessment path, with much more volume than others. Which doesn't make any sense.
Well, it doesn't make sense that it would have a different assessment path--but this is Amazon we're talking about. Obviously, something is different, or the time discrepancy you mention wouldn't exist, right?
The other alternative is that all books really are as slow, but we make changes in book 1s more often and under tighter time circumstances, so that's the one we notice. One time, I had a new release in a series and put every book in the series on sale. All the price changes seemed a little draggy, and they all completed at about the same time.
Also, I've noticed a lot of variation in how long it takes a price change to go live. If it were a regular systemic problem, I don't think there'd be that much variation. It could be a random glitch, or it could be a traffic issue. It's hard to distinguish those, because increasing traffic may make glitches more likely. I've long thought that Amazon, despite its web hosting business, doesn't upgrade its own server capacity as fast as its business grows, particularly with regard to backend issues.