I don't think OS X is a good example of your point because resizing windows was incredibly sluggish for a long time. I always figured it was somehow due to the combination of live resizing plus ubiquitous compositing, but I never understood why it was so slow.
I don’t think the graphics hardware was always “insanely fast” back in those days, but even so, there must have been some terrible bottlenecks in the software.
As a user there was no way around it, and if I recall right, even as a developer it was hard to get decent resizing performance out of the system widgets.
One of the very few times I can recall where Apple has shipped something with such poor performance. Maybe most people didn’t notice or didn’t care because they just don’t resize windows very often?
OS 8 and 9 were winXP like in terms of desktop effects - no soft anything.
Remember, "Aero" was a thing almost 20 years ago and that was a graphically intensive window renderer at that time. Nowadays we could have 50x better effects without a problem but UI designers are not willing to.
Then I switched to mate-terminal which put a nice big scrollbar in the right-side border of each window, so I didn't need that script any more.