I'm no master Game Master and I haven't played with a vast range of GMs, but I've seen enough to form an opinion. I believe that one of distringuishing characteristics of a good GM is an appreciation of timing in roleplaying.
The GM is in control of the narrative pace of the roleplaying story, and thus for maximum impact they should time the climax of the story, force slow points, create a buildup. More importantly, the GM is in control of time in the campaign world itself.
Roleplaying games are conducted in an odd sort of vacuum- they are conducted in a mutal shared imagination space. When no form or structure is imposed, no consistent form or structure will exist. Part of the onerous duty of creating structure is shifted from the GM's shoulders by the Roleplaying system- some of the 'physics' of the world is removed from the GM's burden. But no system I know of removes the control of time from the GM. Unless the GM or one of the players forces time to pass the consensual world coasts to a halt. Indeed, without input from the GM the world gradually darkens, details fade into sketchiness and the Players are left in darkness: the consensual illusion dies.
There is no point in tying Real World time to game world time. Simple actions can take a long time to explain. The duty of simulation requires mechanical resolution which is time consuming.
This need not be a great burden- perception is king. It may take 30 minutes to resolve a short combat, but it can feel to the Players, like 30 seconds have passed if the GM creates the illusion . How can a GM create the illusion of rapid movement in combat? Here are some simple guides:
Make sure that all players are familiar with the mechanics. The first couple of combats may be slow, but Players usually pick up the mechanics quickly. After that dice rolling should take less time, and more importantly, it should take less attention. If this is not the case, simplify the system. Extract the important details and discard the rest.
Make sure combat follows a well known cycle. For instance in my AD&D campaigns the process is simple: I call the start of the round, everyone rolls initiatives, then I announce the enemies initiative (or initiatives), then I call for everyone below a certain point to proceed. Then the enemies get their turn, followed by the remaing players. The round concludes with a quick check that everyone has performed their actions. Once again, a known routine means that players have to pay less attention to the mechanics.
Encourage players to react. Don't make players wait until the end of a description. Encourage them to interrupt. Let *them* react. Let Players/Characters interact with their environment- the GM- as quickly as possible. This may get noisy with lots of active players, at which point the GM may need to quiet everyone and individually poll the players, but active interested players are your goal.
Players reaction in the real world should guide Character reaction. If a player quickly starts to describe an action, this should reflect on the speed of the Characters reaction. Players should be encouraged to be forthcoming with their actions without prompting. This also discourages people waiting for other players to react before deciding their action- which may actually take place before the first player reacted.
Imagine a typical scenario- warrior and wizard in combat against bad guys. The wizard announces they are about to detonate the area with a fireball. The warrior says that they will back off so they don't get scorched. This is fine except for the fact that according to the dice the warrior reacts first...
That scenario is fine if the warrior and wizard talk to each other, but that itself may alert the enemy. Far more often I've seen psychic connections between Characters which permit them unnatural coordination.
Is this a problem? After all, they are heros. Well, maybe it isn't a problem, but it does begin to break the illusion. After all, how far can Players roll back the time flow? Can they take back actions which failed?
Worse, this sort of nebulous time flow means the players lose the illusion of time: time becomes nebulous. GMs need to be clear about time passing. They need to be absolute: an action once taken is irrevocable (unless permitted by some quirk of the game system). Say it as you play it.
Think of how aware we are of time in the real world: if an hour slips by without me noticing I get pretty alarmed. I've seen days and weeks slip by Players without their notice. And I've seen players waiting for a few of the Character's days to pass while the GM is busy planning the next few seconds of the Character's life- they get imagination whip lash as the GM suddenly informs them that their players are still on the bridge, and the bridge has just washed away...
Time is also one of the GM's liberating tools. It is awfully boring for players to be left out because their character is seperated from the players. Here, the GM can compress time until something interesting happens to the lone player. Then they can continue that thread in parallel with the main plot.
My favourite tactic in this situation is to exploit the long Real Time cost of combat to splice together a long main plot with a slow minor thread.
For instance, a single character is sepearated from the main party. He is imprisoned for weeks while the main party come to his rescue. In real time the main party leave the city that evening... and two weeks of the lone character's time passes. The main party travels for two weeks to the destination... while lone character is in single combat. Half an hour later the main party arrive just in time to see the single combat conclude. At no time during the sequence are the party and character in sync, but in Real Time, they are all busy. Also, I find it can add real drama to the rescue attempt to know that the character is in mortal combat and that hurrying really makes a difference...
The above comments address what might be considered technical issues: they are closely tied to the mechanics of roleplaying. Without the imposition of dice, Roleplaying system mechanics and the GM's descriptions, the above comments are more or less superflous, they are specific to Roleplaying.
More general comments can be made about narrative structure in Roleplaying. And it is here that truly excellent GMing can show itself: I believe Roleplaying is akin to a Novel or a Movie in Narrative pacing, and the GM must appreciate the fact that timing is important.
That means that sometimes weeks fly by with little or no comment. Descriptions should be made, but player intervention is not expected (although it should always be encouraged: let the players take up part of the duty of invention). Typically some segments of a journey can be conveyed with statements like 'time passes, and you arrive at...' The party are well provisioned, experienced and the path is safe. Why mark the passing of each day?
Sometimes it is important: you may wish to make sure the players don't take travel for granted. They are carrying valuable cargo and they are expecting attack. Even if there will be no attack a good GM can keep them in suspense by drawing them through some of the common details ('so, who is on first watch?', as the GM rolls dice).
At the other extreme in combat seconds can may be ticked of one by one. In combat it is even more important to keep Real World time ticking over. Make sure that, during combat, Players are enthusiastic. If the combat is routine, then mark it as routine and move on. 'You meet a couple of Orcs, but make short work of them.' Otherwise combat becomes dull.
Pacing in the capmaign world needs to reflect timing in the Real World. As a session draws to an end finish on an upbeat, or a cliff hanger. Try not to leave players in depressing or boring segments of the journey. In the middle of a long and boring sea journey is bad: in the middle of combat with a gian sea squid is good. Take a note from Fantasy Novels: authors rarely end a chapter without some noteworthy event, or some speculative thought which makes you want to come back to read the next chapter... A GM needs to do the same.
Finally, adventures need to have climaxes. I've been in campaigns where the characters stumble from misadventure to misadventure with little or no resolution and the experience is depressing. Adventures need firm climaxes and resolution, however temporary, so that the Player leaves with an Achievment in mind. The Players Wins. Even if the Character dies in the next session, the Win will stick in the Players mind. It is Achievement, Accomplishment and an Adventure the Player can recall. Never ending journeys do not make good stories.
In the end, good Timing is about having a good Time. Good luck.