The faster the hardware gets, the more software becomes inefficient. Unfortunately, hardware hasn't become much faster outside of niche cases (massively parallel vector computations etc.) for the past decade or so.

the GUI code for the earliest macs was absolutely a great feat of engineering

by getting my hands dirty with the system, I'm impressed by the way things were designed for the wimpiest processors (and also getting a lot of insight into the flaws of the OS that were exposed as machines became more powerful)

You can do wonders if you know what you're doing and avoid the web stack.

Funny, to me this machine as described is a monster. My brain still registers a 68040 as a powerful processor. My Atari ST ran a full GUI in 8mhz and 512K of RAM (+192KB of ROM) on a 68000. The original Mac had 128MB onboard RAM. GEOS on a C64 runs a convincing GUI in under 64K... on a 6502!

My first PC was a 50mhz 486 with 8MB RAM, and I ran X11 + various things (netscape, xemacs, etc.) on it without a whole lot of issues.

With the exception of (high-resolution) video, I still do much the same things I did on my computer 20 years ago. Most of it involves fetching and sending text over the internet. It's a little prettier now, the protocols are more advanced, and (crucially) it's a lot easier to develop and distribute applications[0], but in the abstract sense there's absolutely nothing new and nothing older machines couldn't handle given the right set of protocols - i.e. less JSON, less downloading raw application source code and expecting to run them in less than 100ms.

[0] The web is good, actually. It sucks, but it's also pretty good.