Dungeons will take longer than you will probably like at the start of the expansion. This will mainly be down to people getting used to having to use CC again. So a lot of time will be wasted on explaining what to CC and applying the marks.
Once everyone is familiar with this method of play then it will become quicker as you can assign CC roles at the start and then motor through (as long as you have someone fast at applying marks).
If you remember back to TBC dungeons then Shadow Labs or Shattered Halls took a long time to begin with. Once everyone was familiar with the pulls then hitting the Shattered Halls timed event for the extra loot was a regular occurrence.
I suspect that you are going to see some LFD horror stories at the beginning as people try to get away with WotLK tactics (or lack thereof), and fail hard. So, put together guild runs and knock off those guild achievements at the same time
