In my case it's extensive anecdotal experience.
First when I was a budget constrained student with bad hardware.
Of course now I use a really good midrange phone, good laptop and pc.
Then i noticed a stark differences in attitude in the 2 studies I did at uni.
One was more plain compsci. The other more web/mobile design and development.
The later paid absolutely 0 attention to optimisation. Whether it was speed, download size or anything of the sort.
The former treated an inneficient slution as invalid or at least graded lower.
(Note this was in Beligum)
First time I started paying more attention to this where it wasn't when it wasn't in the context of being a budget constraint student was when I did a temporary IT support stint at a gigantic international pharma company and the software was so sluggish it was....insane.
Dozens of people at our facility staring at a slugish webinterface waiting at every action and doing loads of those for every ticket.
It was so infuriating I tried to make a browser plugin to prefill stuff with comon defaults the moment they were available knowing I'd never use it beyond those 3 months. Years of time must've been wasted staring at little loading circles because management did not use the software.
Then when I made native software for a manufacturing industry niche where i was told some suggested db retrieval and prerendering optimisations and such weren't worth the time....And sure. These were lowwage foreign workers using it, it didn't hamper much at all and our time was expensive. Maybe the business wouldn't get the money out of it....but then i ventured to production and notice more than a hundred people pulling the same actions many times a day thousands of times a year ....and see them waiting split seconds or multiple seconds depending on the actions that counted up to combined wasted minutes, hours, days, weeks and I realised what a load of bollocks it was.
Additionally when I helped extend various ERP's I realised the old ones were faaar from great but the move to web based interfaces really didn't do us much favours.
Then I come home and try to help my various layman family members on their comparatively slower windows and android devices and it becomes veeery apparent that this is not just a professional software issue.
Slugish UI libraries on slugish frameworks on a slugish OS with various other slugish apps.
Coming home to a fast pc running Linux and using stuff i write in my free time using C++ or rust or even just various native apps on windows feels like such a extremely stark contrast.
My conclusion is that users won't care enough up to a certain point as studies show
However I do believe that what those studies don't show is that for a good part this is because it's the default for many of them.
They think this is just what tech is and all those countless small waittimes behind actions is normal.