I'm a bit rusty on this but from memory the overhead is by and large specific to the Win32 environment. Creating a "raw" process is cheap and fast (as you'd reasonably expect), but there's a lot of additional initialisation that needs to occur for a "fully-fledged" Win32 process before it can start executing.

Beyond the raw Process and Thread kernel objects, which are represented by EPROCESS + KPROCESS and ETHREAD + KTHREAD structures in kernel address space, a Win32 process also needs to have:

- A PEB (Process Environment Block) structure in its user address space

- An associated CSR_PROCESS structure maintained by Csrss (Win32 subsystem user-mode)

- An associated W32PROCESS structure for Win32k (Win32 subsystem kernel-mode)

I'm pretty sure these days the W32PROCESS structure only gets created on-demand with the first creation of a GDI or USER object, so presumably CLI apps don't have to pay that price. But either way, those latter three structures are non-trivial. They are complicated structures and I assume involve a context switch (or several) at least for the Csrss component. At least some steps in the process also involve manipulating global data structures which block other process creation/destruction (Csrss steps only?).

I expect all this Win32 specific stuff largely doesn't apply to e.g. the Linux subsystem, and so creating processes should be much faster. The key takeaway is its all the Win32 stuff that contributes the bulk of the overhead, not the fundamental process or thread primitives themselves.

EDIT: If you want to learn more, Mark Russinovich's Windows Internals has a whole chapter on process creation which I'm sure explains all this.