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

I'm gonna guess here that the biggest chunk is the antivirus. Turning off Windows Defender's protection(s) should give the first visible speed boost, if that's what you prefer.

Another big chunk of this likely happened when they hardened the graphics subsystem for security. Win32 user calls are unbelievably expensive nowadays. SendMessage etc. have a ton of overhead.

Another chunk is likely the sheer number of expensive DLLs that need to be loaded and initialized with most apps. For example, IIRC, the moment you load COM or WinSock DLLs, your app stops loading snappily. Pretty much anything will load COM even without intending to.

Another chunk is IMM - the ctfmon process you love, for multi-language/keyboard support. ImmDisable(0) can make loading a bit snappier, but then good luck with keyboard switching and the like. It uses window hooks, which are slow Win32 calls as mentioned.

People think it's just a matter of writing plain Win32, but that's not the whole story, although it certainly helps compared to more heavyweight frameworks.

> Turning off Windows Defender's protection(s) should give the first visible speed boost, if that's what you prefer.

It's extremely hard to do that in recent versions of Windows. The most I managed to do the last time I tried was to disable it temporarily but it always comes back after a while.

A couple of years ago I was doing a mass copy of files from one SSD to another. It was a few hundred GB, not terribly big on modern machines but it did have a large number of tiny files. Windows was doing the copy but it was estimating that the whole thing would need 8 hours to complete, and the estimate was pretty solid after 20 minutes. I cancelled the copy to investigate and tried turning off Windows Defender (but only temporarily as you said) and restarted the copy. It finished in 35 minutes. Probably would have been even faster if I didn't have one of the drives hooked to an old USB->SATA adapter.

This is also why your browser will stall out when it finishes downloading a large file. Windows Defender kicks in an does a full scan before returning from the close call.