Part of how you keep encounters brisk is by grouping NPC behavior into as few actions as possible. As a rule of thumb, NPCs should have no more actions than there are PCs in the encounter. Even though you'll likely be interacting with PCs as the NPCs act, you'll still have PCs sitting on the sidelines for tens of minutes in between the NPCs turns. The longer they sit idly by, the more quickly they become uninterested in what's happening.
That means if you have a BBEG, a boss, a special, and 5 minions in the encounter, it may be necessary to make the minions act as a group so you're only taking (at most) four turns in the round as the GM.
Who Should Be Cannon Fodder?
While the section on encounter composition suggests that only minions should go down in a single hit, it doesn't hurt to apply this rule to specials as well. The threat NPCs pose to the PCs is not in the amount of HP and AP they have to sustain damage, but in their ability to deliver damage across multiple actions in the round.
Alter Context, Not Stats
While it's cheating to alter NPCs stats on the fly, you can alter the circumstances of the scene if the encounter is taking too long.
- Skip Ahead. If it's obvious to everyone at the table that the party is going to mop the floor with the NPCs in the encounter, there's no longer meaningful choice in the encounter. Don't let everyone stand around kicking dead horses. Let the PCs decide what happens via a single-act scene check to tease out complications. (Perhaps success at a cost could suggest an NPC escapes capture, for example.)
- Reassess NPC Morale. NPCs are only interested in fighting as long as it serves their interest. Very few monsters fight to the death. Surrender or flight are always an option for them, so you might automatically shift their morale when it's obvious the battle is not in their favor.
- Bring in Reinforcements. On the contrary, if the PCs are unexpectedly trouncing the encounter because you messed up its composition, and there's room in the fiction to have the NPCs receive reinforcements, bring them in. You should only ever do this sparingly, as you don't want to rob the PCs of feeling competent or make the encounter longer than it needs to be.
- Introduce Twists. The old adage, "Rocks fall, everyone dies" (which supposedly has its origin in the original Tomb of Horrors) these days embodies the sentiment of a beleaguered GM who wants to end a scenario in frustration. A well-crafted twist, however, can alter the trajectory of the encounter with some fairness if it's rolled randomly. For example, if a battle with hive monkeys seems to drag on, perhaps all that commotion has awakened the Hive Mother, a colossal monster that plans to steamroll everyone in the scene. Now the encounter has transformed into a chase. It doesn't hurt to keep tables with random twists like this handy, so you can reach for them when you need to spice things up.