It's not the rendering, it's the mechanics behind that would be sluggish because all of XIV's interface issues are due to the fact everything has to go through the server rather than the typical Client > Server > Client. It's more like Client > Server > Server > Client, every action has to be registered with the server then you need to get a response from the server for that action then you get the desired outcome, which is why they've mentioned needing to optimize the servers still even though they're stable excluding market wards. It's not an issue in animation rendering as that's the whole purpose of CT lol, it's more on the mechanics side of thing.

CT is more of a cinematic engine, they've admitted it was a mistake to go with this engine initially, (http://www.finalfantasyunion.com/new...take--855.html), since the engine was more designed as a supplement, cinematic and compatibility engine, it's not an adequate gaming engine. Even recently UE got seamless transitions going efficiently, whereas CT had it just prior that update. So CT does the job, but it's main focus was for cinematics.

XIV is horribly optimized, a lot of it is due to the choice of making the interface serverside which iirc, was to be done as a "RMT countermeasure", every update the response time gets a bit better, but there's still a noticeable delay which isn't present in any game that uses client side interfaces. Another example is Active/Passive mode, ignoring the animation lock, you can get a noticeable delay in switching when the server is having issues, whereas in XI, even if you have animation delay/lock, your client sent the info and the server confirmed it and you're already attacking even without seeing the attacks, in XIV? You need server responses for everything, that's why swimming mechanics would just be..sloppy based on the current game architecture.