Seems Samsung EVO SSds experience a slowdown when working with old data and have released a fix for the problem. I ran it and it took about 40 minutes on my 250 GB Samsung EVO SSD. See here:
Samsung delivers fix for SSD slowdowns
Samsung delivers fix for SSD slowdowns
One of Samsung's most affordable consumer solid-state drives is experiencing read slowdowns with older data, a problem the company has now addressed with a firmware upgrade and installation guide.