I don’t know if this is a bug or intentional but on Supernova and Maw of the Void, if you delay clearing the initial area to get your base, the timer on the Merc compound will have already started, so you can get units out of them sooner wrt enemy waves and objectives.
On the other hand, in Media Blitz, the Merc compound timer only starts once the initial sneak attack period is over (or the Odin gets destroyed).
Especially useful on Supernova, if you wait about 4+ mins before clearing the canons, when you get your base, the War Pigs are immediately available and the Fire Wall will not have come any closer than if you speedrun the opening section. This has the advantage of being able to get two sets of each Merc out before the original compound is destroyed, though you may not have the resources to actually get them. You can even get the third set of War Pigs out before it burns down, no need to build another (unless you want more mercs and have resources to get them). I had 4 Siege Breakers and 4 Dusk Wings out by the time the starting base was burnt down. Which was perfect for the ‘no Barracks or Factory units’ 10th anniv achievement.
I hope it’s intentional, or a bug that won’t be fixed 
Though, would like the merc timer to start sooner for Media Blitz and others, so we can have more mercs out faster.
Maps with an initial section before getting your base but doesn’t start the merc timer early: 
- Dig
- Engine of Destruction Can Tychus even be slowed down if we hang back?
- Media Blitz
Maps which do start the merc timer from the mission start: 
- Supernova
- Maw of the Void