Windows NT on 600MHz machine opens apps instantly. What happened?

Some additional things to note:

Windows NT 3.51 minimum hardware requirements were a i386 or i486 processor at 25MHz or better and 12MB of RAM for the workstation version. So the 600MHz machine with 128MB RAM is exceeding the minimum requirement by (conservatively) 24x in CPU speed and 10x in RAM, along with all the architectural improvements from going from the i386 to what's presumably a Pentium III-class machine.

If that's actually a Surface Go 2 running Windows 11 - well, it doesn't have a quad-core i5 as the tweet claims - the Surface Go 2 came with a Pentium Gold or a Core m3; both with only two cores and of those is an ultra-low power variant.

As such, that exactly meets the minimum CPU specification for Windows 11 and only doubles the minimum 4GB RAM requirement.

I'm not trying to apologize for the difference here, but it's not an entirely like-for-like comparison.

I feel like the intent was to say notepad from 20 years ago and notepad from today has (approx) the same functionality whereas the processors are x4 times faster, it should be at least as fast as it was before, shouldn't it? In my mind, regardless of the OS requirements, a processor x4 more powerful shouldn't need double the time to launch the same program unless you've added x4+ features.

Notepad back then could only edit 32kB maximum files, even on 32bit NT, it was literally all the text widget could handle.

So no, it's not really fair to compare a 'simple' text editor.

It is a fair comparison.

If you edit the same 1KB file on each computer side by side the 30 year old computer will be more responsive than the modern one.

That's what people are taking issue with.