In the handful of games I've had the opportunity to Play the most time consuming parts were not the actual gameplay. The pre-game terrain designation and setup take as long as, if not longer than, the 6 turns of the game.
Defining the terrain will most likely speed up once we have played enough games with our FLGS's stock of terrain that we have a general understanding of what each piece is classified as. We have been using the competitive terrain placement rules as well, which slows things down.
The back and forth of deployment is also currently time consuming, as is battle card selection when used. Half of the games I've played so far have been from Operation: Silent Hail so the Objectives, Deployment, and Conditions were pre-set. Again, I think these are things that will speed up with experience.
Without having the Tournament Regs for Legion it is impossible to know how "Turn 0" will affect gameplay and round length. With Runewars, terrain placement is a part of setup just like in X-wing and Armada. However, none of those games use custom terrain as much as Legion, if at all.
If FFG decides that terrain placement is a part of the game they want to include in the competitive ruleset then I think one could, possibly, make use of a chess clock or other timer for terrain placement and unit deployment since Setup is not usually included in the time limit for each round.
Personally, I would prefer to have each table set up with terrain that is pre-defined by the TO, as the scenario for each round will most likely be. This would eliminate any discussion mid-game about what a certain terrain piece does. It should also eliminate any potential for dispute about fairness of the tables and rounds since all players would have the same setup and a piece of terrain that one set of players decided provides heavy cover isn't determined to provide light cover by another set of players.
I don't think that chess clocks would contribute to actual gameplay. Legion is an objective based game. Playing to the objective, knowing that the tie-breaker is points defeated, and even accounting for opponents that are perceived to be playing slow can all be considered part of being a commander, which is what a wargame is really about. How one accounts for all of these factors and deals with them is what differentiates between poor, average, or exceptional commanders.