There wasn't supposed to be a Part II, but I thought of a few more things people can do.
These are a bit more "helpful" and less "grinds the game to a halt if you don't do them" kinds of things.
Know Your Crippling Thresholds. Know how much damage it takes to:
- blind your eye
- cripple a limb
- cripple an extremity
- score a major wound on you
Know Your HT, Death, and Consciousness Rolls. They're all the same by default, but know them and what exceptions you've carved out with things like Fit, Hard to Kill, Hard to Subdue, or Berserk.
Know Your Large Area Damage DR. It's an average of your torso DR and your lightest DR exposed to the attack. Yes, if you have an open-faced helmet this means you use that zero, at least in my games. This one will vary, but not as often as you think - lots of times all that matters is "I'm facing the explosion" and "I'm not facing the explosion." But it's a pain to have people fumble with this one when an explosion hits an area and everyone needs to know.
Know Your FP and HP Multiples and What They Mean. Know what "below 1/3" amounts to for these, and what that does to your guy. The others are easy - at -1 x FP you're losing HP instead, and all the HP multiples are -X times HP. Those are easy, but it's disappointing to have someone say "Oh wait, I'm at 4 HP and I have 13 HP, so I should have been at half Move and Dodge for like, three turns!"
Know Your Encumbrance Break Point, If Any. If your Move and Dodge go up if you drop your shield, or drop your backpack and go to battle load, know this ahead of time. No one wants to sit around while you figure out if you drop your potion bag or get that bedroll off if that's enough to drop your encumbrance.
Like the previous bunch, many of these are solved by a good character sheet (we use Phoenix from GCA, it's pretty good). Or you can just write them down on paper like I do for NPCs. Just make sure you know them.
Some of this surely makes GURPS sound complicated. It's not, just detailed, and those details are very individual. It's only slow when people don't know their character's details, or what those details mean. These and the last bunch are the real biggies in my experience.