I have no issues with the GCD time as it stands. It could be taken as a somewhat inexperienced opinion, as I've not played much of MMOs (only one I did invest time into was XI and XIV 1.0, and the times were far longer in those two), but it's mostly because I don't feel any need to constantly be pressing buttons to be entertained. I like being able to simply look at the battlefield every now and then when I don't have anything to use. I also don't have any problems having to wait 3 or 5 or even 10 seconds to cast something. I'm not in any rush to start doing something else, and I definitely don't find it boring to be waiting for it either.

As a related aside: I play all of the roles that are available, and I've not had any issue with the GCD on any of them; that includes healer, tanks, melee and ranged (both caster and physical).