Firefox: Grinds to a halt after my 2nd or 3rd attempt, but recognises each one I (sort of, as it stuttered) drew accurately. Chrome: Stayed smooth but did not

Firefox: Grinds to a halt after my 2nd or 3rd attempt, but recognises each one I (sort of, as it stuttered) drew accurately. Chrome: Stayed smooth but did not recognise a single digit correctly. Not sure what causes either of these problems :(