I'm not sure why human reaction time is relevant here, since what I'm talking about isn't the time it takes me to respond to a stimulus but the time it takes the computer to respond to a stimulus.

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.

I was curious, so did a quick web search, which claims that 300ms is the average reaction time and plenty of people run faster than that.

But I think the question was the other way: Why couldn't calc.exe launch in 300ms?

The Doherty threshold is 400 ms. That’s the threshold which you start impacting users focus, and flow.

Back in the day, we actually used to aim for that as a user experience metric.

yeah no. Ask musicians using computers - 50 milliseconds of latency between sound and movement is generally considered unplayable, 20 milliseconds is tough, below 10ms usually is where people start being unable to tell.