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

Well of course it does.

WinNT 3.51 was released in 1995 - the fastest PC in 1995 was either a Pentium or Pentium Pro at ~100 MHz - in 2000 a 600 MHz machine is likely a Coppermine PIII.

A fairly common amount of RAM in 1995 to Run WinNT would have been around 32 megs of ram, 64 megs would be especially generous. 128 megs is a high end workstation amount of memory.

The ATA interface also doubled in performance between 1995 and 2000.

There were significant security and stability improvements between NT 3.51 and Windows 2000 - particularly with changes to the driver model that increased stability. (even more so between 2000 and Windows 10/11)

That's right, Windows 95 on a 600 MHz machine would be even snappier. However, later down the thread the author demonstrates Windows 2000:

>For those thinking that the comparison was unfair, here is Windows 2000 on the same 600MHz machine. Both are from the same year, 1999. Note how the immediacy is still exactly the same and hadn’t been ruined yet.

https://twitter.com/jmmv/status/1672073678102872065