Why are cavalry units so slow and weak?

[Vincent Butler]

if you want real combat cavalry as Romans, you should use Equites Extraordinarii for charging and the other two only as a support for them. With a tier 5 regional MIC in any Gallic province, you can recruit Brihentin, which are very well balanced multi-purpose heavy cavalry and much superior to Equites Romani. There's also Numidian and Cantabrian mounted skirmishers in their respective regions, and Illyrian light lancers (both merc and levied). Then there's mercenaries.
After the Marian reforms, Equites Gallorum are probably your best bet. They're an improved version (more armour while retaining the same amount of stamina) of the Curepos that Quintus Sertorius keeps plugging.

I never got to Marian reforms, my campaign went slowly because of the need to send troops back for retraining, even with a Gallic army, sometimes all you get are Celtic slingers, archers, and levy spearmen, so not much ability to create a powerful fighting force, at least not very quickly, I rarely got to a level 5 MIC, too much money and time. Also, I could never get a general with the right traits.