Ever since I started running the Serenity RPG, I’ve been using the following rule for actions in action turn/combat round/whatever you want to call it.

Each extra action above the first comes with a -1 die step to the attribute. Say you have a d8 Agility. If you changed a magazine on a gun, then want pop off a shot in that turn, you can.  But the shot is a d6+skill.  Say you double tapped, the second shot?  d4+skill.  While doing that you were snagging your rucksack and wanted to get up.  Roll either an Agility or Strength and Athletics (if you had a Strength of d8, you would roll a d4+Athletics, as opposed to a d2+Athletics for an Agility.)

The limitation:  once one of your attributes is at d0 — you can do nothing, including take any kind of passive defense or passive perception tests (you tunnel vision on your tasks.)  We find it works out to roughly 3-5 actions a person could take in 6 seconds, each increasingly harder.  I’ve found that most players will leave themselves a d2 or d4 for wiggle room, should they need to defend against attack or notice something important.