I don't think it's fair of you to switch to the language "ruining things for others" - of course they don't want to explicitly "ruin things".

If you had said, "I don't think making sure that things don't happen instantly is explicitly part of their grand plan" then I would dispute that statement. (Yes, making sure things don't happen instantly is explicitly/implicitly part of their grand plan.)

They want users to have an "experience" not just click a button or press a key and instantly see the result. It is very explicitly part of their plan, and if the algorithms don't slow things down enough, they actually request animations or loading screens that do so.

Some of it may be implicit, rather than explicit. For example, a front-end javascript framework might actually produce pixel for pixel the exact same result as a simple HTML file generated more quickly on the back-end might do, but could still be a preferable "experience".

All this could be tested explicitly. For example two groups of PM's could evaluate web sites, where one included Working... screens where a javascript framework slowly renders something client-side, and the other could have pixel-for-pixel the same thing served from the server, using less bandwidth than it took to even transfer the javascript files over, so that the result is served much, much faster.

I hereby formally predict that some PM's will prefer the slower version for the exact and only reason that it is a slower user experience, and that this could be verified with a double-blind and carefully controlled experiment. I've put my claim out there, and it is easily falsifiable as well as has explanatory power in terms of the software we actually see.

Very simply: some PM's prefer a slower user experience. (I claim.)