Could you explain what the objective quantification of UX is?

Clicks per task. Search time. User errors. For starts.

Here's a start: http://measuringu.com/essential-metrics/

Isn't his just moving the goal post? Now the subjectivity is choosing what metrics to measure.

Are 10 easy clicks worse than 2 hard clicks? Is a long, easy experience better than a short stressful one? And so on. Couldn't that vary by each person's preferences?

By trained or by untrained people? Domain specific experts, or newbies? .And why "clicks"? For many tasks, the way to get them done realy quick is with keys...

I had to do some IT support for a doctor's office many years ago, and (unrelated to what I was doing), had a chance to look at the system the front desk lady used to manage the patients, appointments etc. and it used a "clunky" old text based system (perhaps even DOS based?). TO me, it looked utterly bizarre and incomprehensible. But man, the front desk lady was lightning fast doing anything with it. Of course she trained on the system for years, and just entered keypressed rapidly that didn't make sense to me, but achieved the desired result... So, zero clicks, low search times, no (for me visible) user errors... so I guess they never should have switched, as this was the "objectively optimal" UI?

Personally, I don't think so. I.e. I think the UI could have been better, and easier to learn. But of course it's also important to see how usable it is once you do learn it... I was at that doctor's office again, not so long ago, this time as a patient... new lady (the old one retired, I assume), new system... flashy graphics... everything done with clicking, I don't even know if there still are shortcuts, the new lady certainly didn't seem to use any. And it did feel slower overall -- though this is of course a subjective evaluation only, with many years between the two observations. Plus it's somewhat unfair to compare somebody who trained a decade on a system with somebody who for all I know started a week ago... but I guess that's exactly my point here...

I listed starting points. There's an entire science for this. https://en.m.wikipedia.org/wiki/Human%E2%80%93computer_inter...

It sounds like you'd find it interesting.

If UX was fully objectively measured, wouldn't Gitlab, Github, and Bitbucket all look and feel the same?

If performance was objectively measured, wouldn't BMW, Mercedes, and Corvettes all perform the same?

Yes, performance can be objectively measured but different companies choose different metrics to optimize. The choice stems from subjective preferences. Hyundai optimizes for price and caters to the middle class, whereas Bugatti optimizes for speed and caters to whatever their market is. So subjectivity is inherent in the metrics we choose.

... and you have your answer to the prior question.

Derailing the intent of your parent comment and then taking the higher ground...good job bud

An objective measure of metrics does not necessarily imply there is a single optimal solution.

You shouldn't make design decisions from case studies alone, but the subjective experience of each individual user is a perfectly valid measure of UX performance.

No it's not. That is completely against the idea of ux. What someone says is not what they do. Measure. Analyze. Experiment. Tweak. Results. It's all objective data.