Okay, two things about this:
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There's nothing new about software rot (≝ an unchanged program no longer working because something in its infrastructure has changed), a technical term to which I was introduced in 1965.
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I think you may be misattributing the very real slowdowns you are observing. My understanding of the situation is that there is a longstanding competition between the hardware developers, who are trying to make computers faster, and the software developers, who are trying to make computers slower by inventing things such as window systems and artificially intelligent user interfaces that drain processing power from the actual applications. I've been watching this competition for 60 years now, and the software developers are consistently winning; computer systems have gotten slower and slower. When I was a high school teacher in the early '80s, one PDP-11/70 processor could support around 20 connected terminals without the users complaining about lag. Today the vastly more powerful processor in your pocket can barely support one user. (The PDP-10 systems I used in the late '60s to mid-'70s, which were more powerful than the PDP-11 but still puny compared to my current phone, could support more like 100 users.)
The people who write advertising for the hardware companies are as dishonest as you say, but the engineers, when talking among themselves, are pretty clear about the capabilities of their systems. They tell customers about "gigacycles/second" when "cycles" aren't commensurate across architectures, but in the technical literature they use units such as "gigaflops/second," which are comparable across architectures.
P.S. To be fair, we ask and expect more from our computers these days.