Game carts also have much faster access times than just about anything else contemporary because they're just ROM. If Linux, OpenGL, Vulkan, etc. were in the ROM of your Steam Deck, and your games were on ROM (not flash) carts, that'd boot up instantly too.

Windows 2000 boots up fast on modern hardware. You're looking through rose-colored-ass glasses if you think it booted up that quick on the hardware available at the time of release. Windows NT was a pig in its day, but at least it was a clean pig, free of spyware and other unnecessary crapware (unless you were like a client site I visited, and just let Bonzi Buddy, Comet Cursor, and such run rampant across your sensitive corporate workstations).