It was, once.

Nowadays (for about 12 years already I think) there is nothing much stackless about it.

The concept was nice. Stackless and greenlets.. yess. But the way they rewrote C stack just killed caches. Even a naive reimplementation just using separate mmapped stacks and wrapping the whole coro concept under then-Python's threading module instantly gained like 100x speedup on context switch loads like serving small stuff over HTTP.

Edit: Though at this point it didn't much differ from ye olde FreeBSD N:M pthread implementation. Which ended badly if anyone can remember.