From an OS POV both of those tests are doing almost exactly the same thing: make a system call, have the kernel spawn a new unit of execution (whether it's a thread or a process), wait for the child to be scheduled, have it terminate, wait for the operating system to notify the parent process of termination. There's a little extra bookkeeping in this example for the child process, but not much at all thanks to copy on write. If you were to do something dumb from the child work like memsetting a 1MB buffer to 1, I assume the child process would be far slower due to page faults.

There are certainly advantages and disadvantages to both threads and processes, but it's not really a fair comparison to claim that processes are as fast or faster than threads because you can spawn them at a certain rate. The performance cost of separate processes is something you pay gradually, every time you have to take a page fault and copy 4KB.