Same is true on ships. It's easy to say AI should build faster ships. But the AI doesn't script out what technologies it should get. The reason the aI doesn't currently tend to have faster ships is because it doesn't priorities getting faster propulsion techs enough.

Not exactly true.
For example in my latest game, all the AI:s have warp drive. I even got the technology from some other major.
This doesn't mean that they always put them on their ships however. In my experience, they research propulsion technologies well enough but they don't use them much.
I often see escorts with two warp drives. I sometimes see frigates and cruisers with one warp drive but more often than not they don't have any extra engines severerly crippling the designs. I have never seen any heavy fighter with engines. Transports have always only had one engine (isn't it possible to add two so that they have the same speed as the escorts?).

A followup to my last post. I seem to have gotten some things wrong.

This is now my understanding of how the AI uses his different military shipclasses:
Tiny:
Fighter: No engines. Attacker.
Small:
Defender: No engines. Defending planets.
Heavy Fighter: No engines. Attacker.
Escort: Multiple engines (often see them with two). Defending transports.
Medium:
Frigate: 0-1 engines. Attacker.
Cruiser: 0 engines (at least I don't remember having seen any). Attacker? Defender? Maybe both?
Large:
Battleship: 1 engine (haven't seen any with no engines yet). Attacker.

No engines on defender is IMO fine. I have no opinion on no engines on fighter because I never build those. Heavy fighter would benefit from an engine though since the AI otherwise wouldn't have any fast attack ships before Medium hulls. Cruisers are okay as planetattackers if they have faster attackers screening their advance, would also be good defenders, otherwise probably not much use.
Now frigates should IMO always have one extra engine but they don't.

Here are the Drengins different generations of frigates that I could see in my current game (he has lots of old ships left as you can see):
gen. weapons. defense. speed
M2-2. missile+2.
M2-11. mass+2. 1*warp=+3
M3-4. mass+3
M4-1. beam+6. armor+1. 1*warp=+3
M4-10. mass+4. armor+1,shields+1. 1*warp=+3
M5-3. mass+5,beam+2. shields+2
M6-2. missile+8,beam+4. shields+2
M7-1. missile+12,mass+1. armor+2. 1*warp=+3

Sometimes he skips the extra engine in favour of more weapons/defense which is dumb if I'm correct about cruisers, because then it becomes identical to one.
I've noticed that the Terrans in the same game builds incredible many frigates, almost exclusive. This makes them IMO much more dangerous than the Drengin whom also builds lots of heavy fighters and cruisers. Even though the terrans military is much smaller.
And the terrans frigate M7-2 is so much fun
mass+24 (3 nanorippers) armor+2 speed+3
Almost as good as my best medium hull, the Death Dealer:
beam+24 (2 psyonic beam), armor+2, speed+6

I also noticed something odd with Drengins latest transport M5-3. It has 3 ion drives, 3 adv. troop mods and 1 extended support. It could have had 3 warp drives instead of the ion drives because they are both size 10 on a cargo hull which would have made his transports 6 parsecs faster per turn.