Quantum computers, it has been claimed, could perform some computations much faster than classical computers. Such claims require us to compare the performance of nonquantum (“classical”) and quantum computing. In this presentation I first discuss some examples of performance-comparison problems encountered in practice, arguing that the problem of performance comparison exists even in non-quantum computing environments. To help address those issues, including the question of how we can compare the performances of classical and quantum physical computations, I propose a model-theoretic necessary (but not sufficient) condition of physical-computation performance-comparability that is consonant (but not identical with) with several “syntactic” accounts of computation in the literature.