Ed wrote:
'There is a specific time when the second curtain starts its movement only when the first curtain has uncovered the entire frame ... that is called the "mechanical" shutter speed...'

Well, I've learned something there. I've never heard that term used for that meaning. Is it common? Not knowing the correct terminology I'd just have called it the 'synch speed', and I'd have used 'mechanical speed' for a speed that doesn't need a battery. They may, of course, be designed to be the same speed.