Initiative is a score that player rolls for his character where his margin of success and failure determines his order in initiative.
Initiative is Tactics roll with SPDx2 OR IQ. If the character does not have Tactics, they can default to -6, Strategy-6 or Soldier-4. Combat Reflex adds +1 to Initiative.
In the beginning of Combat everyone rolls initiative.
SPD 6.00, CR, Soldier IQ+2 has an Initiative-11
SPD 5.00, IQ12, Tactics IQ+1 has Initiative-13
Double rewarding SPD.
A smarter person may think faster and be slower in combat, but his physical speed will be reflected in Movement and Dodge NOT in the order in which he acts. Because of Wait maneuver a faster thinking character Waits and uses his faster thinking to Prepare and Predict the outcome. Of course if his prediction is wrong, he gets screwed so it balances out.
So a Faster Character may Act Fast in the sense that in the second he may travel farther and dodge better but it doesnt mean they can bring actions and plans online faster than a smarter quick-thinking character.
In GURPS it is not clear that the "start" of combat, those who have not acted is considered "flat-footed" like in DnD. You can still defend as though you came from a "Do nothing" turn in the very beginning of combat.
I like using the Core Mechanic through out the system when it is possible. Using a 3d6 Check for initiative benefits the bell curve and makes for some random variables in combat.