I've been playing the Arceans recently and they take some getting used to, but they are actually very powerful and fun to play. The key to playing them is that they take some time to get up and running and consolidated. Yes, they are slow, but keep in mind that they have the Weather Control Center which gives them net two more squares per planet. If you have 3 planets, it means that in effect you get another PQ6 planet, with the beauty that the squares are on already developed planets (you benefit from the bonus improvements). It takes a bit of time, however to build the nav centers and the WRC's, so you will need to make a best effort to stay out of war in the early game.
As far as speed is concerned, once WRC and and Subspace Drive are researched, and from the planet with the Stellar Forge (speed +1), I believe you have speed of, err, 4. Now, as others have pointed out, it is extremely nice to trade for impulse drive ASAP. Their is usually one civ who will trade it to you for about five techs or 2000 odd bc. With the improvements and impulse you can get speed of 6 very easily, which is the equivalent of 3 impulse drives per ship. Not bad. You usually get this setup long before any civ researches Warp.
I play with tech trading simply because it reflects "real life", that is, ideas and technologies spread from culture to culture as people migrate, become dissatisfied, seek better jobs, or form alliances. Gunpowder spread from China to Europe to the Middle East (IIRC). But even if you don't play with tech trading, the Arceans have another advantage from their setup: Without the engine you can fit other stuff on the ship. Someone pointed out that constructors are light. True. Ships can also pack more firepower, and they have the Super warrior which is sometimes hard to remember to use (you must attack, not be attacked).
Once the Arceans can get consolidated and settled in on their planets they are very, very tough. I can understand how the AI doesn't play them well. You need a bit of early patience but it pays off. It's all about the setup.