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?
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 so it can be done in parallel.
A lot less than that (micro seconds). For example I was trying to benchmark the time it takes to compute merkle root hash of block 692,400 containing 1,121 Transactions using my own code that doesn't have SIMD and it takes 902.3 μs to do that. With SIMD or a stronger CPU than mine it would take a lot less.
| Method | Mean | Error | StdDev |
|------- |---------:|---------:|---------:|
| Merkle | 902.3 μs | 17.29 μs | 14.44 μs |
This amount of time is negligible in comparison.