I'm not sure there's any causality between Windows preventing overcommit and Windows having slow process creation- there's nothing inherently slower about CreateProcess()/posix_spawn() than fork() + exec() as an API.
It seems more that overcommit is a workaround for the way fork() can potentially lead to copying the entire address space into new pages, but usually doesn't. Because CreateProcess() knows more precisely how much to allocate before it returns to the child process, it can just reserve that amount and signal an error immediately if there's not enough memory+swap to back it.
(And on the other hand, Windows has a lot of legacy and backwards compatibility behavior around processes that could easily explain the slower process creation independent of the API.)
It seems more that overcommit is a workaround for the way fork() can potentially lead to copying the entire address space into new pages, but usually doesn't. Because CreateProcess() knows more precisely how much to allocate before it returns to the child process, it can just reserve that amount and signal an error immediately if there's not enough memory+swap to back it.
(And on the other hand, Windows has a lot of legacy and backwards compatibility behavior around processes that could easily explain the slower process creation independent of the API.)