Re: you wouldn't use a cat 5 patch lead and expect 10Gb/s performance
HDMI is a digital* protocol and the performance of the link can be (and often is) measured in the analog domain**. Different materials will yield 'good enough' for various lengths of 'good enough'.
* One of my favourite sigs from many years ago (and still valid today) is "There is no such thing as a digital signal; EMC testing proves this daily".
** A signal is only really 'digital' at the protocol / determination layer (Signal above / below some set of thresholds).