Well I mean in the end, it's the ratio of Fastpass to Standby that are being allowed on. I know once that I counted how many they let on Little Mermaid before letting I think 6 standby through the merge was 30-40 FP.
And that's where the big slowdown comes from. All of a sudden that big standby line has to feed through into a much smaller portion of the ride's capacity. I wouldn't be surprised if one side of Matterhorn becomes FP only and one side stays standby. How can that NOT make the line move much slower?