This reminds me of an old story about the original development of OS X Aqua in the 90s.
At the time they were doing functioning mockups for the Aqua design in Macromedia Director. It was not easy to actually implement the UI with the available hardware, but the mockups showed that it was possible. "Mr. Ratzlaff frequently would remind Apple engineers that 'Macromedia can do it, so why can't you?'"
> As it so happened, Mr. Ratzlaff and his team had already been working on a ground-up redesign of the operating system after they discovered that their original goal of porting the look-and-feel of Mac OS 8 completely to Mac OS X would be impossible. "We were only going to be able to get about 95% of the way there [putting the Mac OS 8 interface on Mac OS X], which is possibly the worst thing you could do," Mr. Ratzlaff said. The redesign had been scrubbed by higher-ups at Apple but was put back on track by Mr. Jobs.
> With that, Mr. Ratzlaff and his team's list of requirements for the operating system, which had been called overly ambitious and led to laughter from engineers who heard of them only weeks prior, became mandatory. These included 32-bit color with alpha channels and QuickTime integration, all being able to run on a system with just a G3 processor and as a little as 8MB of video memory. Mocking up the operating system's design and functionality with Macromedia's Director, Mr. Ratzlaff frequently would remind Apple engineers that "Macromedia can do it, so why can't you?"
I guess it depends on how fussy you are about these things - I definitely noticed my 16" MBP lag at times. When the system was under a lot of load in particular the Chrome dev tools started to get quite laggy.
However I got an M1 and can say this is no longer an issue. (despite having only 16GB RAM compared to the 32 on my 16" also). When using native apps the M1 laptop runs amazingly well, it is one of the best performing machines I have ever used for day to day use, even compared to my 5950X desktop system. Of course the 5950X will run circles around it with highly parallel workloads but on single thread the M1 is basically just as good.
My early 2015 Macbook Pro can be a little sluggish when I have lots of stuff open on my 4K monitor, but I think it'd be a little harsh to judge a six year old machine for that! This laptop's done very well really, it's definitely going to have an afterlife as a media server or something after it stops getting macOS updates because I'm loathe to get rid of hardware that's still doing a fantastic job.
I guess that it is fast because modern browsers are fast, implement good algorithms to repaint just what need to be repainted, and delegate a lot of work to the GPU. Exactly like the Quartz compositor engine.
Also the real computer has to think about it more. (E.g. initialize whatever services the applications uses, be concerned about the data structures the applications mirror.). The demo just has to look right on initial inspection.
