Funny how it can take 5 years to realise something like this; maybe
"revenue" is the key. Microsoft didn't earn significant revenue from
user clicks on its interface in 2012 and was left clueless?
Yet it shouldn't be too hard to notice a 20 % slowdown with small
usability tests/focus groups. It could be interesting to test a couple
existing skins and a couple big interface changes in the works (such as
Special:RecentChanges and Special:Search) to see if there is any such
big gap anywhere.
Would eye tracking be needed too?
I've not read much on the topic, just this and little else:
The only mention I see on the wiki is from Dario in 2012:
I don't remember if Abigail Ripstra was doing something of this kind.