I think I've found a huge flaw in the AI that heavily limits their economic output. A quick fix here would make the AI much more nasty!
The AI never seem to build enough farms! I'm currently playing a game on gifted at turn 150, and invading class 14 planets with just 14 population - it was pop cap limited! Compare to my home world which has 80, or my primary production world's each of which have at least 40.
The lack of population severely limits the AIs raw production and thus economic output. Subsequently their research suffers, they generate less wealth and thus can only support smaller fleets, and they produce fleets more slowly. This makes them much easier to conquer, as I tend to have more, higher tech ships that I produce faster. I win by attrition.