Quote Ben Noordhuis, thank you Ben!
Apache on Windows serves all requests from a single process. The Ruby interpreter is not thread-safe so mod_ruby creates a Big Mutex whenever it needs to run. Thus on Windows, with its single-process model, mod_ruby can only serve one request at a time.
Just for the record, this is how I compiled mod_ruby with Apache 2.2.17 for Windows with MingGW 3.4.5