The miner, indeed, hashes a 80 bytes block header, but to reach the certain merkle root, he has to hash transactions. Hashing transactions may take some time; it may not seem slow in practice, but in theory, doesn't it take more time to hash multiple transactions over and over again, instead of just one?
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There isn't a lot to hash. Anyways, I don't think ASICs hash merkle root. They only hash the block headers, any CPU can hash the merkle tree in milliseconds.