As one of the few who regularly play Noble, I would like to note that the AI tech rate (with a few notable exceptions) has slowed dramatically. The usual suspects (Shaka, Monty, Caesar, etc) weren't really surprising; what is a shock is seeing otherwise decent tech AIs (Wang Kon, Ragnar, Hannibal, etc) getting left in the dust after Feudalism. The only thing that seems to help is if the AI starts with Gems or Gold (or two!) in their capitols BFC. Anything less and I never feel pressure tech wise, unless I expand too slowly.
So what is the difference? There's no bonus on Noble for the AI to lose, so that's out. Espionage is a factor, and not just the slider-- some of the AIs will just send Spy after Spy into my territory, for little or no gain. Over expansion was an early suspect, and may still be true. Units? Maybe. After some trouble with warring in the first few games, I tend to strike much, much earlier now, so I haven't noticed a huge change in the otherwise peaceful civs. I still maintain there's some issues with city terrain development, but haven't looked at a lot of examples closely yet.
I dunno. Something has definitely changed, but what it is- if there's only one or two things- is very difficult to say for sure.