> is it the UI animations, color themes, shades etc etc or is it the underlying operating system that has more and more features, services etc etc ?

...all of those and more? New software is only optimized until it is not outright annoying to use on current hardware, it's always been like that and that's why there are old jokes like:

    "What Andy giveth, Bill taketh away."

    "Software is like a gas, it expands to consume all available hardware resources."

    "Software gets slower faster than hardware gets faster"

Sometimes there are short periods where the hardware pulls a little bit ahead for a few short years of bliss (for instance the ARM Macs), but the software quickly catches up and soon everything feels as slow as always (or worse).

That also means that the easiest way to a slick computing experience is to run old software on new hardware ;)

Indeed. Much of a modern Linux desktop e.g. runs inside one of multiple not very well optimized JS engines: Gnome uses JS for various desktop interactions, and all major desktops run a different JS engine as a different user to evaluate polkit authorizations (so exactly zero RAM could be shared between those engines, even if they were identical, which they aren't), and then half your interactions with GUI tools happens inside browser engines, either directly in a browser, or indirectly with Electron. (And typically, each Electron tool bundles their own slightly different version of Electron, so even if they all run under the same user, each is fully independent.)

Or you can ignore all that nonsense and run openbox and native tools.