As in literature, role-playing games need to have proper pacing. A potentially exciting combat sequence can turn into long, dull periods of inactivity punctuated by a few die rolls. If you want to keep your players interested in your game, make your pacing match what you're doing. Talk is cheap, of course. Being told that you need to watch your pacing is worthless if you don't know what to watch for.
Practical advice is far more useful.
A useful model for pacing is to think of it as information flow. The pace is quick if new pieces of information are thrown at the players quickly, if a lot of thought is required for each one and if important decisions have to be made every time. The pace is slow when your players don't get new information very often, when they have time to mull over the data and if there aren't immediate decisions needed with each change. Consider, for example, a tense combat situation. The pacing here should be breathlessly fast.