There is no canonical reference to the speed at which a lightsaber activates being controllable by a Jedi or Sith, either through use of the Force or through technological means. That includes the EU.
The closest is likely the description in The Krytos Trap, by Michael Stackpole, of an old lightsaber discovered by protagonist Corran Horn in a museum. When Horn turns it on it is described as extending "slowly," with the light produced being "dim" and the hum-buzz being "thready and weak." There is reference to the use of "dual-phase" lightsabers (again by Corran Horn, though Darth Vader also used a dual-phase blade) which are capable of changing lengths, but there is no significance whatsoever to the speed at which lightsabers extend or activate.
Except, of course, dramatic effect. During the scenes you mention, it makes sense, from a dramatic or theatrical point of view, for the lightsabers to activate at the speed they do. If you pay attention, you'll notice that every audible lightsaber activation in the films - with the possible exception of the dozens, if not hundreds of activations during the Battle of Geonosis scene during Attack of the Clones - is done in tempo with the score of the film.