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? https://www.usability.gov/how-to-and-tools/methods/eye-tracking.html I've not read much on the topic, just this and little else: http://www.ep.liu.se/ecp/101/004/ecp13101004.pdf The only mention I see on the wiki is from Dario in 2012: https://www.mediawiki.org/wiki/Wikimedia_Features_engineering/Notes/2012-W07#Dario
I don't remember if Abigail Ripstra was doing something of this kind.
Nemo