In the spirit of the Prismatic Wasteland blogwagon here are a few suggestions for players, that in my experience, avoid slowing down the game. Nothing earth-shattering, but I think it's worth bringing up because these items represent perennial issues that have detracted from the game across generations (I'm sure I could find similar suggestions in the hallowed annals of Alrums & Excursions).
In the spirit of I CAST LIGHT!, the advice is coming from playing 2-3 years of weekly megadungeon-based games that span 2-4 hours each with other busy adults. But even if we had 8 hours and no other responsibilities, the following advice would apply saving time and preventing slow down.
Spells- know them: Know your magic spells- targets, saves, ranges (touch/distance). A sub-species of this is knowning how your class abilites work as well. And no, you won't remember them by just writing down the name.
Most MU classes won't have a lot of options so its not like you'll have to copy several spells down at the beinging of the game. In fact, OSE has a helpful one-page spell sheet. And honestly, if you are playing BX D&D, most any source of spells will work. At each level, you'll be adding only 1 or 2 new ones. Pre-writting your spells becomes even more helpful if you are an illusionist or druid class as those spells tend to be lesser used and discussed. This is also true for spell-like effects like Dolmenwood's enchanter class.
At the very least, screen-shot your DM's book and save them in a folder with your character's name on your phone. Done.
Equipment List- have one: Know your shopping list. Print one out and staple it to your character sheet OR ask the DM for one. Second, only to spells as the tedium that will suck the life out of a game of D&D quicker than a vampire. This is because you'll have a table of 3-5+ people going one at a time asking "okay I need X, is it available? How much? How much does it weigh?." Bleh.
Its uninteresting, especially so since after a delve or two, most PCs can afford all sorts of basic equipment. Its more interesting to understand if they can carry it.
For DMs, here is a good post about the cost of basic equipment using a copper standard to make comparing prices easy. I highly recommend having some scarcity based on city versus country. But also, as a DM you'll want to have costs of silvering weapons, holy water, flammable oil, and if potions/scrolls can be bought.
Downtime- discuss 24+ hours prior: Let your DM know what the PC wants to know in game, prior to the session getting together- not 24 min after the session as started. If you as a player don't know what an option during downtime is, then ask the DM what they will allow. But even if you have a menu to choose from, it still best to let the DM know what needs to be generated. It might take time to generate the answer or an NPC, or perhaps the downtime action might have an immediate effect on the world and so would be best played out immediately.
DMs even if you don't like carousing, its still useful to have a little menu about what PCs can accomplish in a week's worth of non-adventuring activity. Like how long to get armor made, how long to get fresh rumors, how long to ask my MU mentor to look something up, potions to be made, or get that special thing from that city, and all the associated costs.
Goals- discuss them 24+ hours prior: This is a group activity. The players should have an idea about what they want to do and where they want to do it as a party. Is it going to be a typical back-to-the-dungeon session, which the DM might have material for? Or is the party going to chase down the rumor of the Wailing Witch in the swamp to the east? Since you, the player, might not, or should not, know the state of the world, you might not know that rumor is not in a section of a fully fleshed-out world. Perhaps the witch's cave needs at least some hasty scribbles on a Dyson map. Hard to do 10 mins into a session, but easier to do if given 12 hours.
DMs don't be afraid to just say its a dungeon session if you've not had time to prepare for some other thing they want to do. Especially if not given the time to prepare in a manner you feel comfortable. D&D is a hobby where both "sides" need to work to make the fun. Its never enough just for the players to show up.
In summary: players kindly take a little bit of the logistical, decision, and knowledge ownership of the game off of your DM's shoulders and give back to your DM the time to prepare in advance. Yes, your DM is a stellar weaver of adventure, but I can guarantee that whether they do 30 mins of prep or 8 hours, they still would love to have ample time for creativity. And if you have spells in hand and torches, rope, and 10-foot pols already picked out, your next adventure can start out with a bang!
Or at least the news that the local lord's worthless children want to join you on a dungeon delve...