Giving a rubber a certain speed-label depends on your reference. Many reviews lack vital additional information to make the review actually useful.
I'm currently testing both Vega X (1.8mm) and Rakza Z (2mm)on a Hugo SAL. Only played approx. 5hrs so far, so still not enough playtime for a proper review.
Compared to Rakza 7 (also 2mm) the Z is significantly faster. Even on the SAL, the Z provides more speed then I'm used to and prefer. I surely wouldn't put the Z on the my Freitas (which I barely use tbh), as for me, the speed would be way too much.
Compared with some other rubbers I used to play, Rakza Z is faster then Vega X Vega Tour, EL-S, FastArc G1, Rakza 7, Rakza X Soft, Mantra M, Acuda P2.
(all in 2.0-2.1mm except for Vega X (1.8mm))