I've been thinking about this, why would they do this? What if it reduces the load on the system?
I think FP+ was getting gummed up because it had too much work to do. To hang on to your reservation it needs another state for each time slot that has to be negotiated with the central servers. A ride/time could be available, pre-reserved (being booked), reserved, and released (needs to add back to the available pool).
G+ just has available and reserved. Released may exist but there are FAR fewer of these because no one wants to give up a reservation BEFORE booking a new one further reducing load. A reservation "book it" request is simpler, it just gives you a time from the pool without checking the time itself or looking at your account to see what you've been promised.
Each time you make a change to a ride/time the change must be communicated to the central server. Semaphores/locks must be established and shared out to redundant servers. FP+ was doing a lot more of this kind of work than G+ does making it easier to support large numbers of users without downtime under G+.
So this shift may have been intentionally intended to serve the machines better than the guests, and some guests noticed.