It's definitely true. Momentum is incredibly important when it comes to sets, especially tournament sets. Being able to slow things down when you need to is invaluable.

I've often played /u/angelicxx in our local tournaments and I think it's a good example of pacing, as you describe it. Wolverine with a beam wants to rush you down constantly and subject you to mix ups almost non-stop, so he will often play very quickly, and between matches he instantly hits rematch to keep his mental momentum going. I on the other hand play slowly, as a Spencer player without a full-screen beam I can't really afford to play fast and aggressively. From the beginning of the match I'll back away, then super jump, trying to keep Wolverine from starting his rush down game and building up that momentum. Basically the goal is to avoid playing at his pace while setting up my own.

If your opponent likes to rush everything, it's a very good idea, mentally, to go to character select between matches. If you feel like your mind is racing too fast for your hands to keep up, take a break, breathe, hit character select, and think. Another example is when I went to Canada and played Bee in a FT15. I think the score was about 12-12 before I started to stop every match, regardless of the result. I recognized that Bee is aggressive - not as much as a Wolverine player, but aggressive nonetheless - and that if I broke up his pace often he would have trouble adjusting.

The only reason I personally can benefit from these things is the nature of my team. Having Frank on the end and transitioning him to my Point character mid-match means I don't have my complete kit available from 99 seconds; my game starts once I get a clean hit. Most players have their optimal team at 99 seconds, whereas I don't. What this means is I can afford to stop the match often, and always choose to, because even though momentum is amazing for me and my playstyle, that winning momentum will only last until the end of one round - then I have to regain it from zero next round.

I think the same can probably be said of Dormammu and Morrigan players. Dormammu just needs one moment to gain control of the match, and until he finds that moment he's at a disadvantage. It doesn't matter if he just won the match; he doesn't need to capitalize on a previous win, what Dormammu needs is that one play that lets him start his game.

I think characters like this are incredibly potent, and because of them players can afford to stop their pace, and can afford to slow down games or even stop for a few moments. When you play a team like a Wolverine team or a Firebrand team, or even a Zero team, the game is all about shutting down your opponent and preventing them from doing anything, so unfortunately you pretty much always have to be playing at a fast pace. It really depends on the characters you play IMO, which is just another fascinating aspect of the match ups in this game.