I think he meant: Single-threaded entirely gets rid of the need for locking and critical sections (so long as you don't need exclusive access to a resource for

I think he meant: Single-threaded entirely gets rid of the need for locking and critical sections (so long as you don't need exclusive access to a resource for longer than a single callback), and evented lets you recover some of the concurrency you lost by sticking to a single thread (if you're IO-bound and you code carefully). In a single-threaded event framework, even if two external events fire simultaneously, their callbacks will be serialised because there's only one thread to run them in, so it's as if they fired at slightly different times. There'll be some latency processing the "second" event of course, but not too much so long as the callback of the "first" event doesn't take long (hence the "code carefully" part).