What is the best way to create many shifts?

Context: We need to performance test our sync engine which pulls humanity shifts. As part of this we require setup/cleanup.

Problem: When creating shifts we can only create one at at time by providing relevant info, when trying to create many shifts (a few thousand shifts) 429 (too many requests) are returned. Is there a way to create many shifts at once? i.e. provide an object or array of shift properties?