Pacing is hugely important in running a tabletop rpg. As a GM, you have to be acutely aware of the pace of things at the table-- in the big and little sense. In any moment am I managing the speed of things? Are people still involved and interested? How much time do we still have left to play and what can I get to in that space? Bad pacing can lose players-- both in terms of their active interest in what is currently happening and their engagement of the story as a whole. I'll leave off the big picture narrative pacing concerns for the moment and just focus on one little portion about engagement-- combat pacing.
Combat's where you really shouldn't slack off. I say this as a GM who like combat less than most other aspects of the game. However, I also understand that it provides a certain catharsis for the players and a potential source of excitement, tension and drama. No combat should feel like it is a throwaway-- there should be stakes and risks. If it is just a filler, you have to invent some outside pressures-- resources at risk, a time-limit, innocents threatened, etc. I think that as a GM you have to present and maintain the illusion that any particular combat is important. Even if it really isn't, you have to sell it as that.
That means that you need to move quickly from one player to another, and, paradoxically, you have to constantly repeat yourself as quickly as possible. When you come to the next PC, recap what's going on, who their adversaries are, and what the current threat is. If you have three or more players, they've been sitting there for a couple of minutes+ waiting for their turn. Get them back into things quickly and push them to make a decision. After a couple of times of this, they;ll get used to having to move quickly when it comes back around to them. That speed and illusionary quick movement from player to player keeps up the tension and maintains the pace.
When the bad guys go, handle that as quickly as possible. Focus, of course, on the drama, and feel free to give them minor wounds or obstacles happening from the bad guy's attacks. Straight wound systems (Storyteller, Unisystem, etc) don't give greta rnage for this, but if you fudge a damage roll, feel free to throw in some color like unbalancing effects, knock down, bad footing, slight bleeding, etc. NPCs should be done quickly as well. In a combat remember that opponents are literally an unlimited resource for you. Players enjoy taking down a few mooks and it makes actually finishing a tougher big bad guy more satisfying.
One of the important considerations for pacing is that part of your job as a GM is to weave together all of these slices of time, these moments broken into actions or phases, into a coherent narrative. Feng Shui describe actions as "shots" and I think that's a reasonable way to handle it. Even if a player fails, they ought to feel that failure is part of the story. They're the wiffs the hero goes through before getting their act together and finally winning. Think about bad fight sequences from films, where things go to fast so you can't tell what's going on, or moments drag out, or bits are thrown in that don't fit. It is an imperfect analogy, but GMs need know that combat has the same set of structural needs as the rest of the game-- you need to make the scene work. That means being certain and being flexible at the same time. The first to reduce hesitation and slow-down, the second to be able to adapt and shift the story focus depending on what is going on.
As for me, one danger I recognize is going too fast and losing the players in what's happening. Different players will maintain a grasp of the situation differently and you have to get used to that. I'm still not fond of combat, but I need to get better at it. I'd still say it is one of my major gamemastering weaknesses.