I picked combat speed, but actions was a close second. I think my actions concerns play into combat speed more than anything.
I recently got a chance to play a game called
Old School Hack. It was brilliant and fast. I believe there were two reasons it went so fast.
The first was the action schedule. You got to pick one and only one type of action to perform each round:
- Defend or Protect
- Shoot
- Focus/Impede - i.e. start casting a spell or declare your intent to stop someone
- Move
- Attack
- Push/Throw - Throw as in "He throws you across the room."
- Resolve Focus actions
The Fighter had a special "Charge" ability to move and attack in the same round. Otherwise, do the action you declared and call it a round.
The second was movement. There was no grid, you just split up the map into "arenas", that is areas that are big enough to hold a fight. You can move one arena with a Move action.
I can easily see working a version of both of these into DnD, and probably will for my home games if 5e doesn't do something similar in the basic game or a module.
Honestly, though, there's plenty of good choices on that list.