Fastest device to perform single-threaded SHA256 hashing
Well, existing Bitcoin mining hardware would indeed not be any good for iterated hashing, as it's not designed to do that. However, SHA-256 is not a hardware resistant hash function. The most complex operation in the hash function is addition modulo 2³², which is quite easy to implement in hardware.

An attacker could implement a fast SHA core on a chip, which would consist of calculating a single round as fast as possible. This is the opposite optimization as a Bitcoin miner: miners prefer very many parallel cores that run at slower clock speeds to save energy per hash. Such a hypothetical chip would definitely be much faster than a CPU at calculating iterated hashes.

What you want is a memory-hard hash function such as Argon2. Its execution requires fast cache, which is plentiful on commodity CPUs but costly to build into other hardware. It has a parallelism parameter where you can allow up to, say, 4 cores. Iteration of the function will not be any faster past that number of cores, and custom hardware will not increase the attacker's speed.