Why are cavalry units so slow and weak?

[QuintusSertorius]

Also, I know that the cavalry in RTW were unrealistic, but I still think these heavy cavalry units are exaggeratedly slow and tire out too quickly to be useful or worth the insane cost. I'll see what I can do once I'm able to recruit non-mercenary light cavalry.

Unless those non-mercenary light cavalry are lancers, don't bother. Just hire Curepos, who are cheap and available all over the place.