It's not slow. It's just boring. Majority of the excitement comes from boss mechanics and dodging puddles of fire on the floor.

Actual "gameplay" is extremely lackluster/generic. Multiple mirrored classes and no class can bring anything unique or interesting to a party because balance is paramount in a WoW clone. It's not like FFXI where you could have some classes be "weaker" but also have unique utility or aspects that could make them ideal for certain fights. Even if you couldn't pump out as much dps as some classes, you could bring something to fights no one else could and really fostered class identity. Now every dps/tank/healer class may as well be the same, and they say repeatedly that they design content that can be completed with any class composition. That's pretty much required when all you're doing is waiting in 30 minute long queues to go dodge AOE circles and spam a rotation. Sure, some classes may have a skill that slightly increases your critical chance or increases the damage the enemy takes for a few seconds. It makes no meaningful difference in the grand scheme of things. In the end you're just X healer/tank/dps class with a different coat of paint that simply has to learn boss mechanics and satisfy the Recount meter.