My cdr was lacking even with obsidian , but it was okay since I ran Battle Rage-Ferocity instead of WoTB and Ground Stomp only takes a few seconds to reset. 1m sheet damage but with +39% fire and +26% whirlwind, so kill speed was still manageable. The more difficult part for me was the timing needed to navigate the trapped lanes, and to deal with larger monsters/elites. Sometimes they are near a rend group so it becomes necessary to either avoid their melee range or stun them as part of the group, or simply take them out first even if means losing the rend group. I also found it necessary to clear the first open area and first two lanes thoroughly, so there is no need to backtrack beyond the last two lanes for stragglers.
Basically, this dungeon is not terribly time strict but has a higher requirement for precise controls (similar to Earth set).