I've picked up the game again and been playing Hard and the occasional V.Hard AIs on medium maps, and I've noticed that even when set to Aggressive, they just don't DO much.
Early game they're fine - a horde of Cobalts or what have yous comes out and blows me up when I'm not ready for it, sometimes a whole fleet with capital ships, but every time, around the midpoint of the game, and AI seems to just 'turn off'. Like they automatically go into some sort of defensive mode. Very rarely at that point will they attack - they just seem to build, build and build. In a four-player FFA, the two AIs that started together in the second star system haven't done anything to each other, while I wiped the floor with the one in my system, because after his intial aggressiveness wore off, I was able to easily grow, more or less left completely alone, and I was able to conquer most of his systems with little more than a carrier and a few transporters.
This is also probably contributing to the late-game minidumps and massive performance slowdowns - the AIs are just accruing resources, ships, and technology, but aren't depleting each other. Defensively they aren't too bright either - I take one planet, odds are its neighbor is going to be next, but they never move to defend it. In fact, they seem to favor just one single huge fleet that takes eons to move around.
Any chance we can see the AI get a brain makeover at some point?