Time cost of thread creation
Every model of computation has a supported list of instructions, each associated with its time complexity. Thread handling is not a basic instruction in any model I'm aware of, but probably for practitioners of parallel computation, it is. The time complexity really depends on your viewpoint. If you're interested in optimizing the inner working of the operating system, then thread creation is not a basic operation at all, and its cost is associated with the algorithms implementing it. If you're interested in parallel computation, then probably thread creation is a primitive operation of unit cost. It all depends on the context.