Might try it out on my Speccy when it's released because it looks like another nice black-and-white game for that platform, but it looks like it still doesn't have the fluidity that a race game needs.
For me, really the only 8-bit 3D race game that has that is OutRun for the SMS. Or perhaps I missed a game or two?
it is impossible to take advantage of VDP specific features like hw sprites, however i think done in the spectrum way could be done and will be very nice to have
My favorite ZX-Spectrum game is CHASE H.Q., it was really good on ZX-Spectrum 48K without ability to use double screen.
About the game in this topic, for sure they are using double buffer of ZX-Spectrum 128K. Also, if you pre-calculate road and fill RAM with it, you can just copy road during play without need to calculate so it will save lots of time. The top background moving only few lines, so shouldn't take much time too. And after placing background, you will need to place cars and foreground with masks. I think it could be done in 2 (maximum 3) frames. So there is potential to have 25 FPS.