It’s not. It’s either 33% slower than perceived or perception overestimates speed by 50%. I don’t know how to trust the author if stuff like this is wrong.
She's not wrong.
A good way to do this calculation is with the log-ratio, a centered measure of proportional difference. It's symmetric, and widely used in economics and statistics for exactly this reason. I.e:
ln(1.2/0.81) = ln(1.2)-ln(0.81) ≈ 0.393
That's nearly 40%, as the post says.
I find that satisfying.
It’s more obvious if you take more extreme numbers, say: they estimated to take 99% less time with AI, but it took 99% more time - the difference is not 198%, but 19900%. Suddenly you’re off by two orders of magnitude.
Still an interesting observation. It was also on brown field open source projects which imo explains a bit why people building new stuff have vastly different experiences than this.