The difference between stackful coroutines (i.e. fibers in this case) and Python-style coroutines is that Python's coroutines need to rebuild the stack on every

The difference between stackful coroutines (i.e. fibers in this case) and Python-style coroutines is that Python's coroutines need to rebuild the stack on every resume, whereas resuming a fiber is basically a goto. The cost of yielding and resuming a coroutine in Python is O(N) for the depth of the call chain, but O(1) for fibers. So as you say, a simple scheduler that just walks a linked-list could resume a stackful coroutine in a few instructions, plus the cost of restoring hardware registers (which is the same cost as a non-inlined function call), and the latter is easily pipelined so would take fewer cycles than instructions.