So there are a few "knowns" regarding one of the Doctor's most fearsome foes, the Cybermen. They are essentially humans with their emotions removed, encased in a fully metallic body, hardwired like a computer, a very tactical and strong opponent, and quite slow and clunky... that is, until they decided to terrorise an amusement park at super speed. In the episode Nightmare in Silver, the Doctor brings Clara and the children she watches over to the Moon in order to visit the largest amusement park in the galaxy, which happened to be closed for business due to Cybermen interference. Damn. However, these weren't typical Cybermen as they seemed to move with the speed but thankfully it only lasted the duration of this episode and the Cybermen have been back to their typical slow clunky selves for all of their subsequent appearances. It's still not known why Moff decided to speed them up exactly, but it was not received well by many fans. That's probably what helped him reach his decision to stick with the more traditional approach. If it ain't broken, don't fix it.