Pacing is to do with the player's sense of "continuing action" throughout the game.
Some definitions of pacing:
"The player's sense of passing events -- actions, decisions, plot points, puzzles ... -- the sense that things are happening or that something is going on"
"The rate at which the player encounters new game elements,
as compared to the rate of his (logical) actions -- where a logical action may consist of several actual commands"
- Andrew Plotkin
"The quality in which the narrative unfolding or the
modulated frequency of events is geared towards an aesthetic effect"
- Steve Breslin
Pacing and Puzzles
Puzzles are often used to control the pacing of IF games, mainly because they require thoughtful decisions from the player. Andrew Plotkin writes in this RAIF thread, "Anything the player has to stop and think about doing is effective pacing."
Players have reported a difference between having to go away and think about a puzzle (in which case the game feels "suspended" and the pacing is unharmed) and feeling "stumped" by a puzzle, which can make the pacing grind to a halt. (See this RAIF discussion thread).
Pacing and Plot Advancement
Emily Short points out that pacing is also affected by actions that irreversibly alter the game state:
"If a player realizes he's making an irreversible decision or plot-advancement, this tends to affect the perceived pace of the game ... people complain about being asked to advance the plot too frequently or when they haven't been given enough time to investigate the surrounding environment."