In some cases yes, in other cases no. It all depends on how often calculations have to be made, how much iteration is required, and how much it calls on heavy functions like pathfinding and distance calcs (and even then, it can be mitigated with cacheing). It is of course a limiting factor, but the biggest problem in making an 'adult level, tougher, smarter, creative ai' is actually writing said AI. You want a 'creative' AI? How exactly do you define creative? The AI cannot come up with strategies of its own, those strategies have to be coded into it and then tested, tweaked, and re-written as they inevitably fail to take any of a thousand different situations into account.
Something people fail to understand about AI is that while the AI can make a million calculations a second, a lot of the things you take for granted as a human is not available to a computer: Intuition, imagination, improvisation. A computer has none of that. It can only work with the data it's given, it cannot infer where data is missing, and while this may not sound like that big a limitation trust me when I say that it is.
For example, let's say you're France and you're gonna invade Spain. A human can look at the ledger, see that Spain has 100k troops, that they're at war with England, and that they've recently lost a lot of transports and infer that a significant part of the Spanish army is probably stuck outside of Iberia. A computer either has to be given some form of algorithm to calculate the expected number of troops in Spain (which would quickly grow very complex) or simply cheat, peer through the fog of war, and get a number to work with. In short, for something that any human familiar with the game can do in two seconds, you either have to put down a huge amount of work or let the AI 'cheat'.
Now with that said, there are some rather nasty army AI bugs in the current version that are getting sorted out next patch, and as usual any big patch will contain a bunch of incremental improvements to the AI.