Freescape

To clarify: I am still impressed, but at least now it "only" feels like really good programming, when I thought that was on a 3.5MHz machine it looked like dark magic

Oh, wow, really, that's at 3.5MHz, then I go back to my impression of "dark magic"

Confirmed; I don't know why it's distributed as "the 7Mhz version", though I could take an educated guess*, but running that version in a regular Spectrum emulator produced output much like that in the video.

* compared to the '14Mhz' version in the same archive, draw distance is reduced and the number of [sort-of] textured surfaces is reduced. So I think the Mhz ratings are level-of-detail recommendations. Both seem to be frame-rate independent in terms of movement.

hard to say without looking at the code. It depends on how does it access video memory. If it uses a double buffer, it should be fairly straight forward, if it does any fancy tricks directly on video memory, then A LOT of work.