I suppose Maxpass is too new and not a lot of people know how to use it properly. There were a lot of confused people in the park last week when we visited. The most important thing that I discovered was that you can begin making your selection BEFORE rope drop, that is after your tickets have been scanned for the day and before the official opening time. I managed to secure FastPasses at a very early hour last September 15 when Holiday Haunted Mansion opened for the first time. So while everyone was rushing to the ride reaching 90 minutes mere moments after opening we very casually strolled other rides which were completely empty.
Since you can also make new FastPass selections after your return window starts this allows you to accumulate a lot of passes very quickly but your mileage may vary in DCA which seemed to be a FastPass black hole.
Another protip is NOT to use you smart phone to scan your FastPasses. Use your physical ticket. You might think this is old school but it is more efficient. This way you can conserve battery and avoid holding up the line because you are trying to open the app and adjust the brightness settings.
Sadly, I found that the touring plans are not very efficient with the new Maxpass system. The plans assume you will be around a certain area of the park to pick up the FastPasses and that the return times happen much later in the day. However, the truth of the matter is that since you can choose which attraction is more convenient then you can end up in an area of the park that is completely opposite to that of the plan.