Notes are essential aides for memory over the course of a long narrative. They help one plan sequences of events, track what has happened, and who is involved in the story. However, many DMs use the least efficient and least useful form of notes possible - long-form descriptions of events, sometimes even session by session. These notes are hard to refer to in play, difficult to index, and rarely have all the relevant information required. They also encourage book-flipping, which is perhaps the single slowest and most agonising common activity at a roleplaying table. Long-form notes encourage railroading and rigid plotting, perhaps not intentionally, but simply because they cause one to think that one is writing a story, or at least the outline of a story. Finally, they take forever to make, both in preparation for a session and after the fact, which contributes to referee burnout.
My suggestion is to change the way you make and use notes. Different notation styles will allow one to spend less time prepping and recording, less time flipping through them for relevant information, and allow one to improvise deviations from them more easily than long form notes allow.
This entry, the first in a series, will focus on improving the notes one takes for monsters and antagonists. While not the most absolute and dire problem here, this is a simple, easily remedied source of slowdown at the game. I'm always surprised to see how many referees just pull out the Monster Manual or equivalent and flip between the various entries relevant to the fight, often without even using an impromptu bookmark. Then, having found the monster, further flipping is often required to look up what its abilities / feats do, and how they modify a given roll.
I have several recommendations here:
Get ahold of monster stats in electronic form whenever possible, or create electronic copies of these stats when possible, and print them out. I tend to use the same kinds of monsters multiple times, especially common ones like thugs, skeletons, etc. At the start of a campaign, if there aren't stats for the most common kinds of opponents, I will whip them up, print them off, and use the same sheet over and over. If the monsters use weapons, I will calculate the two or three most common kinds they use, including one ranged weapon, and write them on the sheet so that I never have to flip over to the weapon tables. I organise these sheets with paper clips into encounters, so that I have everything at hand for any given situation, ready to go.
If the monsters have special abilities or situational bonuses, I write out what these do in point form, especially if there's a calculation or trigger that needs to be remembered. If it will come up a lot, I precalculate what the bonus or roll will be, so that I have it on hand, easy to remember. If the monsters have two abilities which combo with one another, I will write the combo down: "1st: save vs. paralysis 60ft 1 target 2nd: Laser shot +4 100 yards 1 target 1d10+3" so that I don't have to hunt across the sheet to find stuff. I make a little list of these on the bottom or side of the sheet, wherever I have room.
Finally, spells. Spells, especially in D&D, take up the most room and require the most hassle. I suggest drawing up one electronic master spell list (two if the game has an arcane / divine split). This spell list should be the spell list of a master wizard (a 20th level wizard if you're playing D&D) with good stats (so bonus spells added already), and have a good selection of spells, avoiding most of the highly situational spells.
Then, whenever you have a spellcaster, you print a copy off, take a black marker, and just scratch lines through all the spells the monster does not have for whatever reason. In most versions of D&D, there are only a handful of spells a smart character will take unless they expect something unusual anyhow, so it makes sense for monsters to be using the most common and effective spells. Most PCs never catch on to this, and even if they do, I've never really found one who cares.