And the important point is: it _is_ annoying, an actual bootup in 5 seconds is easily possible from a technical perspective, and would much improve the user experience.
Take systemd, I remember it brought a lot of startup time improvements for a while, or at least promised them. I don't follow Linux very much these days, but when I do a standard installation of a popular distro - sloooow startup. Last I looked I had an issue with tons of virtual serial devices created serially, took probably 10 seconds alone.
Parkinson's law at work.
But to your actual point, of course you can write software that runs not slower, or even runs much faster, than the software of 30 years ago that ran on machines of the time. The exception proves the rule (while I feel it's always been so obvious that there was never a proof needed).
Not all software is getting slower faster than hardware is getting faster.
It has always been possible to write slow software.