On a second thought I realized that process_iter() uses a global
variable, so it's not thread safe.
That means that if the are 2 threads using it, the first thread one calling the function (+ consume the iterator), will "steal" the processes of the second thread.

psutil.cpu_percent() has the same problem. That means we have a problem
can't solve with the current API and requires a lot of thinking on how
to solve it as it's not obvious.