A physical Z applies a microwave pulse to the qubit.

A virtual Z is a book-keeping trick that changes the rotation axis of later gates.

When you're writing an algorithm, you tend to want as few operations as possible so you'd prefer a virtual Z. When you're doing a physics experiment, or trying to decouple from slowly-varying noise, you tend to want to actually affect the system and so would prefer a physical Z.

I am a little rusty on this but rotations around the z axis on a Bloch sphere can be implemented by either detuning the qubit frequency with respect to the drive field(real z gate) or as a composition of X and Y gates when introducing a phase offset in the field with respect to those gates(virtual z), which is why it takes no time since it's implicitly part of the other operations.