You have to keep the stack around in memory, as opposed to having a fixed structure. And fixed structures really pull away when you think about allocation: you can use a segregated fit/free list structure, whereas you can't with a variable sized thing like a stack. If the stack starts small and grows, you're paying the costs of copying and reallocation whenever it does: another loss. Allocation in a segregated fit scheme is an order of magnitude faster than traditional malloc or allocation in a nursery and tenuring.

For these reasons and others, nginx could never be as fast as it is with a goroutine model.