The thing is that it's more than just the GCD and rotations. FFXIV is a very flashy game with elaborate animations. You can weave in oGCD's sure, but FFXIV just isn't a game that feels all that snappy even at its best. Trying to double weave doesn't really fix that, if anything it can highlight it even more.
I think it's very valid for WoW players to feel like FFXIV is slow. It's not an inaccurate observation and I think people take it for granted these days. FFXIV is a very methodical game. You memorize fights in an attempt to minimize movement and squeeze in as many button presses as you can. Fights are very much paced around rotations, you can even time them to down to the GCD. Some are convinced this remains very frantic, but for others it's almost a zen-like exercise.
I'm never going to fault WoW players for thinking FFXIV is slow. Some people will never acclimate to FFXIV's gameplay and I can respect that.