For one thing, an Assyrian Villager with Jihad and Wheel researced is actually faster than a regular HA. So if speed was changed, HA should be made as fast as HHA because a Villager definitely shouldn't be the fastest unit in the game (HHAs are the fastest).
Making HHA faster than HA makes the upgrade more worth it in my opinion, and it seems logical since this unit's purpose is quick hit-and-run attacks. Speed is important to other cavalry as well but not as important as it is for HAs, so it doesn't change with other cavalry upgrades.
It all depends on the unit line. Most of the time HP is increased, but sometimes it isn't, as with Ballista and Helepolis. So I wouldn't say HAs and HHAs need to have the same speed just because other unit lines retain their speed throughout the line.
Edit: Just remembered that Merchant Ship is faster than Trade Boat. It's actually the same speed as HHA, though HAs are slightly faster than Trade Boats. Here again, speed is increased because it's so central to the functioning of the units.
[This message has been edited by Highwing (edited 06-19-2017 @ 01:32 AM).]