Very difficult hashing function?
Argon2.

Argon2 is the current standard for password-hashing schemes, which are designed to be slow and more importantly expensive to parallel adversaries, even if they have hardware support. You can tune its speed via the memory and the time parameter, either of which should affect run-time linearly. Anyways, the paper in the repository should contain a guide on how to experimentally configure the parameters for your application.