I ran a game at the weekend and some pacing related issues arose.
The PCs descended into the crypts under a town, which I imagined as being quite extensive. I didn't play it out room-by-room, corridor-by-corridor, partly because I didn't have a map of any kind, not even a rough one. I hadn't anticipated the PCs would go down there so to some extent I was making it up as I went along. There were two encounters - a ghoul ambush, and a meeting with what I called 'grey ones' (dark creepers which I imagined hailing from a Shadowfell-like place deep underground), which ofc ended in a fight, PCs being PCs. Most of the intervening parts were pretty much glossed over.
Even if I had had more detailed information, most of the crypts are devoid of interest, so I wouldn't have done a room-by-room, I'd have just said something like, "after exploring a dozen empty chambers, you come upon [insert interesting bit]". D&D heresy.
It was 4e, so 'grind' might be regarded as a pacing related issue, how to keep the fights from dragging. There were three proper fights, which I think went on for about 45 minutes to an hour. We used maps + minis and we are all fairly unfamiliar with the 4e rules, which slowed things a bit. And yet I thoroughly enjoyed each battle. I thought they staying interesting throughout. A PC went down in two of them, which really helps to keep things tense and exciting. So I think an area where pacing issues might have arisen was problem free, at least from my perspective.
One of the players hadn't really played a rpg before and she had a refreshing tendency to try totally unexpected things. She was blinded by a dark creeper and asked if she could use her Insight/Religion to use the experience to get an insight into the nature of these weird creatures. It was cool to run with her ideas. Say yes, as they say.
Could this 'say yes' concept apply to pacing? If the players are really into exploring a room say, should the GM run with it? I'm not sure, it can be a problem if something is interesting for some players and not others. Also I think it can be a problem when the GM knows that an activity will ultimately be anti-climactic. The players think they are heading toward an objective but in reality they are not.
I've noticed that some players can be dissatisfied if an activity doesn't lead anywhere while others don't mind, being happy with a chance that it might. I experienced this playing in an investigative game, in which none of the lines of investigation we had followed so far proved fruitful. One player was unhappy that we were 'failing', but I saw it as being fairly realistic - each avenue of investigation only has a small chance of bearing fruit, and you have to try them all.
This was also an issue in the game I ran. The players are actually down in the crypts on something of a wild goose chase. They are looking for a missing boy, but he is not, in fact, down there. Nonetheless they seemed to be having fun fighting undead and interacting with grey freaks. They got quite excited by the concept of the crypts in the first place.
The game was intended to be a oneoff but is now shaping up to be a multi-session adventure or short campaign. Between sessions I'm planning to add a lot to the crypts to make them more interesting. One possibility would even be to have the boy actually be down there after all. Now that's really saying yes. Even when the players are wrong they are right!