Hrm, pacing is a topic that seems barely covered, so it's good to see a thread about it. I assume you are talking about the speed of story advancement.
1) Personally I don't mind little story advancement during a session. However, no advancement at all feels a bit like wasted time. Yeah, it's all about fun - but the feeling of advancement is an important part of the fun to me.
2) As a GM I try to avoid pushing my players into the action. If everyone stops talking and awkward silence emerges (luckily a thing of the past), I will make sure something happens. Otherwise I give them as much opportunity as possible to keep going by themselves. Which can result in a rather lame session, but they will notice and be more active next time.
As a player, it's more simple: I will start to act, even it's not always wise to do so. There is an old bridge, looking dangerously fragile, and nobody dares to just enter it? I will.
3) As a GM, I usually have to slow down my players when they finally encounter the local boss. They are eager to get into position and talk to them, so I sometimes have to rewind their positions and let the villain do their speech, ignoring the PCs' comments for a moment. I try to keep it short (a sentence or so), but I think any important villain deserves their speech, so I will enforce a certain kind of pacing here.
the pace is too slow if none of the players are engaged. If they are however, but aren't directly dealing with your main plot, it's okay, gently steer them in that direction if you so desire and let it just happen.
Rushing the players can lead to confusion and loss of enjoyment