[staticint]

A mutex held for up to 10ms on each iteration, over 500 iterations, averaging out to 2s doesn't seem surprising at all. In fact, this is quite expected. It would take considerably longer if the random sleep hit 10ms every time.

If you modify the code to release the lock before sleeping...

    func (c *Counter) add(ch chan int) {
            c.Lock()
            tmp := c.Num
            c.Unlock()
            tmp += 1
            time.Sleep(time.Duration(rand.Intn(10)) * time.Millisecond)
            c.Lock()
            c.Num = tmp
            c.Unlock()
            ch <- 1
    }

...and reacquiring it after, it still finishes in 10ms on my machine, as you'd expect. As you can see, the problem isn't so much that locks are inherently slow in Go, but rather the mutex, as the acqusition function name implies, is doing what it is supposed to do: Lock. What is true is that you have to be careful to use them properly.