This is an interesting question, and I’ve noticed that I’ve been having trouble answering it.
After some thought (and reading the excellent discussion so far) I realized that I have a different perspective on this. For me, the choice isn’t between “explosions” and “slow burn”, but between having dramatic issues in play at the table or not.
Is something significant at stake, early on - even if it’s just being hinted at - from the start? That’s what I want, even if it feels subtle or slow moving at first.
In terms of pacing or style, I tend to prefer “slow burn”; the opening scenes of a typical game I am running might be fairly slow, thoughful, or exploratory. However, I want there to be a clear dramatic focus and some kind of important issue on the table right away. In that sense, the intention behind strong opening scenes is closer to “explosions”.
So, the surface presentation is “slow burn”, but the subject matter is closer to “explosions”, if that makes any sense. I aim for an emphasis on the “burn” rather than the “slow”, in other words. There might not be explosions happening or violence, but the slowness is suspenseful and intense, rather than meandering or aimless.
Thinking on it further, in many games I play, there are multiple protagonists or storylines. In that case, I also aim for a variety: if one character’s opening scene is ponderous and suspenseful, maybe the second character’s scene is fast furious, whereas the third one’s is upbeat and comical. This contrast helps keep the interest high and helps us explore different aspects of the setting, its themes, and characters.
Overly “explosive” beginnings work well in other media, but I’m not too fond of them in a gaming context, where I find it takes some time for everyone to enter the imaginary space, inhabit characters, and “feel” the game.
However, this may vary dramatically from game to game, as well; in an OSR context, for instance, I believe it’s usually best to leave pacing in the hands of the players, so I will present the scenario and then sit back. The players themselves might then choose to start abruptly or to move slowly - however, most often, there will be some significant planning, discussion, or downtime, until an objective is chosen and play starts to ramp up in intensity.