There were those apps on pentium 133mhz era computers too, even started in faster tha 5 seconds.

For 99% of the people, word 97 was good enough then and is still good enough now to do whatever they need. They can literally still use it today. (yes yes, docx compatibility, i'm just making an example).

Web stuff is one of the few things where the amount of useful data on an eg. weather report site (webmail, news site, bus timetable site,...) hasn't changed, the cpu requirements have gone way way higher for no useful reason (except to serve more ads), and the users can't use the "old versio" of the site anymore. It's literally gigabytes of ram and multiple seconds of 100% cpu usage to service 5kB of article text and a 50kB article photo.

There were full-on CAD packages for 133MHz Pentiums.

Yes, and they had perhaps 1% of the features you'd expect these days. Though of course, they were the important 1%..

This isn’t unsolvable. A CAD app or similar could ask for permission to use extra resources. There’s already one for storage, extending this to CPU and MEMory is not far fetched.