The medium is the message. In some domains, slow and clunky feels like gravitas and reliability. Enterprise software. Enterprise recruiting software. AWS has a pretty slow, and clunky dashboard. AdSense is clunky. A lot of the biggest companies' flagship apps have a very bland "bureaucratic" look and feel. You may find it hard to believe, but Zuck wanted facebook to look like a government database for a long time, wh…
Slow and clunky is also unreliable, it's did it go through? , did I click on that? Is it loading, or is my connection having troubles? Is the browser again? Some of the ugliest Gov and Gov adjacent pages (e.g. tax collectors, some DMVs) are fast and prompt zero second guessing. They just work.
The slow and clunky I referenced don't have "loading semantics" issues, they all show loaders, action statuses, and many indicate connectivity and save progress for multipage sagas. But you are right that slow and clunky can be unreliable, but so can slick and snappy. Tho, neither of them have to be. In some domains, slick and snappy can feel insubstantial and off-key. It's interesting that the pages you reference are "consumer" products, not enterprise, so that's inline with how I think this goes. I think basically we're talking about orthogonal kinds of slow and clunky, yours is more the engineering focus of how it works, and mine is, like I said in the parent comment, about the aesthetics and the medium in a domain.