Swarmkeeper
Hero
Finally, players have a responsibility to other players. It just makes a game better when the PCs interact with each other, exalt each other, and work as a team.
Yes x100!
In combat: Be ready when it is your turn. A big part of that is knowing when your turn is coming up.
Expounding on the "know your character" theme, one thing that also really slows things down is not knowing spells. I certainly don't expect everyone to know all their spells by heart, but creating a little cheat sheet can save a lot of time at the table. Roll20 has a great spell interface so everyone can see the spell description in the chat. I'm partial to having actual printouts nearby and have used this site a lot - both as a DM and a player - to customize a spell list: Spell cards, PDF. Spells index and Spellbooks for D&D | DND-Spells, The tool for Dungeons and Dragons 5e