Well, yes and no. You're correct that ZX Spectrum is a lot slower than the author estimates, but there's also a software-side performance bottleneck: the code is written in the built-in BASIC language, which on ZX Spectrum, was not only interpreted at runtime, but also almost comically unoptimized. For example, people would use "aftermarket" circle-drawing routines that performed 10x better than the stock ROM implementation.

BASIC on ZX Spectrum was meant for learning, but no serious software of the era relied on it for anything that needed to run fast.

I suspect this code could be 10x faster if written in assembly or a compiled language (a handful existed for the platform).