Why are cavalry units so slow and weak?

[athanaric]

They tire out very easily, I didn't like that either. By the third charge, even if I hit and pull out, hit and pull out, they are already winded, and that is just Equites Romani, ditto for Generals and Hippeis.

Equites Romani and Hippeis are moderately useful for chasing routers and killing skirmishers, so I'm not saying you shouldn't recruit them (after all, they're easily available), but 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.

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

Actually, Armenian cavalry are quite good regardless of armament, so I'd recommend using all of them plus Scythian Riders and Lonchophoroi Hippeis. The only problem is when you expand east, beyond the recruitment range of those units. The two units you get from most of Iran aren't bad, but they can't stand up to the superior cavalry of Pahlava/Saka/Baktria in a straight fight.