Take a quantum circuit on $n$ qubits, you have some sequence of gates.
You can represent these gates as hermitian matrices, and then with some padding, you could take the product of these matrices, by closure would be a hermitian matrix, a quantum operation. Then you have who knows how many gates into one single, but complicated $n$-ary gate.
On a perfect fidelity machine, this could be o(1) right. Does this logic make sense? I feel like it shouldn't be independent of the depth, or a lot of classical circuit problems become constant time on a QC.