I do do still have both computers set up side-by-side (legacy data from an old business), and the keyboard in question was a Microsoft Comfort Curve 2000 (the calculator button wasn't a proper key, it was one of those squidgy extra keys so beloved of multimedia keyboards, so not as fast to operate as a proper key.)
Anyhow, the point (arguably hypberbolic as it may have been) wasn't about reaction time per se, it was about the older calculator app - and by extension much of the rest of the OS - being a much simpler and less bloated piece of software, and running it on faster-than-contemporaneous hardware makes for a sense of immediacy which is sorely lacking in today's world of web apps.
I'd be very interested to know to what that 300ms "threshold of perceptible delay" applies. You might not notice a window taking 300ms to open - but I'd be willing to bet that when you're highlighting text with the mouse or dragging a slider, you'd be very aware of the UI lagging by nearly 1/3 of a second.