I tested the different dragons in 164 in skirmishes on normal speed, against an AI set to 'None'. I timed how long each took to (almost) destroy the recruitment centre (so I could try it multiple times). It didn't seem to make a difference if I kept the overheat bar low, high or fired rapidly until it almost overheated and let it cool almost all the way down again (ie the cooling rate doesn't depend on the level of overheating), though obviously to get the lowest time you need to end up being almost overheated.
The mountain dragon takes about 37-38 seconds to take out a recruitment centre, the sabre dragon about 46-48 seconds and the zephyr dragon 51-52 seconds. Since the building hit points are consistent, this indicates the relative DPS of the default dragon attacks.
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