Slower than SD. No Joke.
It takes longer to render CGI material in high def. If you’re growing impatient waiting for your revised shots bear in mind that the size of 1080 format frame is close to the size of 2k film. HD frame is significantly larger than SD frame.
Frame rate also needs to be considered. Rendering 1080 59.94i takes longer than 1080 23.98p because of the frame rate difference. One format runs at roughly 30fps while the other runs close to 24fps. 6 extra frames can add up!
Any non-linear editing systems that offer real time HD capabilities are a real asset if you’re going to be supervising a session.