Except overall speeds of mountain stages aren't down and there's not really a big noticable difference between fast ridden stages and slow ridden stages in terms of climbing performance.
Speeds on final mountain climbs are up but no one said overall stage speeds are down nor it is about fast ridden versus slow ridden stages
Here is a simple illustrative example of what was meant
Imagine a stage that is 200 KMs
you ride first 40% at 30km an hour
then 60% at 50km an hour
it takes 6.4hours
as opposed to a a stage where you ride
first 30% at 25km per hour
next 50% at 30km per hour
final 20% at 60km per hour
it takes also takes 6.4hours but is a very different race
The first example has a high speed for longer but does not have the very fast final speed at the end .So if you are a fast rider but not able to go hard for long then example 2 would suit you better