Flaming is never something moderators can predict, only react to. Whats more, is that one reason it takes some amount of time for them to intervene is because they are not just called to settle disputes, but settle them as fairly and as impartially as they can. This requires time, thought, and personal deliberation amongst themselves and as individuals.
I know this is true because I have been there and done that on different types of sites over the years. It's a thankless job as you constantly work behind the shadows and sometimes even keep things from happening before they have a chance to or anyone even realizes there was going to be a problem. We cannot see flames, for instance, as something that have to be squashed...or...extinguished (flames....extinguished...get it?....ok...bad pun).
We have to see things from the perspective of what a moderator is called to do. They're not there to merely solve a dispute but to heal and resolve it between ALL the parties involved. Unless the situation calls for it, immediate action is not necessarily the best option. Many situations, however, are clear cut and we see many threads and users dealt with swiftly because this is true.
However, sometimes a situation can become complicated and entrenched, where it's not just about solving a dispute, but about trying to get the people in the dispute to bury the hatchet, forgive each other, or at least agree to disagree and move on. Being a moderator, administrator, or any other person of authority is often more than the title implies, though I like that moderators are called that, because thats what they're called to do: moderate.
They have to investigate for themselves in order to attempt to arrive at a fair arrangement. These things take time and sometimes careful planning. This doesn't mean the staff doesn't see the matter as urgent. Quite the contrary. It's because it's urgent that it requires careful handling.
I agree though that flames are one of those things that moderators cannot afford to let go for any length of time. Flames should be keeping them on their toes, and rightfully so, as flames are not necessarily about rules being broken, but about users being at each other's throats...something just as bad, if not worse than rule breakage, since that affects the overall environment of the forum the dispute takes place in for an indeterminate amount of time. The flame could last a few hours or a few days or longer, and any bitter animosity generated between the feuding members is a liability for them and everyone later, which is why it's important to deal with flames quickly and try to defuse them, not merely snuff them out.
