I feel like OP was treated unfairly. Even if you are a decently geared/overleveled tank, a healer that does not outgear/outlevel the instance can still have difficulties. So many people seem obsessed with things like pulling wall-to-wall or DPS numbers (on a non DPS class) that they forget that not everyone is as progressed as they are. Personally I feel that if your style is always sprinting forward and wall to wall pulling without seeing if your healer is able to keep up with it, then you're not a very good tank. People seem to be defending the way this tank played in this situation when really they just displayed poor empathy for their party. Like especially if there are 3-4 wipes. There is a player behind that healer and they might not be happy with the 25 or so minutes of stress you're putting them through when you could just slow down a tad.

Clemency usage should also be at the player's discretion, not "it's a DPS loss never use it". Guess I should just remove all my GCD defensive/support abilities from my bars because they're a DPS loss, right?