Act first but not actually quicker actions
One of the biggest issues that I considered in all of the games is the normal disparity between having an attribute that is supposed to represent quickness such as Dexterity, Quickness, Speed, or Reaction Speed (REA) and having the ability to actually be quick by doing multiple actions in a single round. D20 products normally consistently perpetuate the lack of actually being in action rather than just being first in line. They prefer the 'I go first' method.
Feat or talent separation mentality
They normally just let Dexterity be added to an initiative roll. What that does is let characters act first, but not faster. A d20 character normally can only act more than once through either a feat/ability - which must be obtained separately - or through a system of standard action, instant action, and full action. While the third is the best to my liking, the actions are normally talent, feat, ability based in which dexterity has no influence on.
I looked into GURPS system and for it you also need to 'buy' more than one attacks.
Three Quickness methods
Only a few actually had anything to indicate speed of actions.
JAGS, Hackmaster, and several d100 systems have the systems that I liked the most. Each accomplish the speed adjustment in combat and round systems differently.
A) Hackmaster attaches the speed to the items, weapons, skills, actions, or spells. I think of it as the utility speed method.
B) d100 has a chart where intelligence and dexterity are combined to see how many actions a character has per round. I think of this as the body-mind method.
C) JAGS to me seems like the most ingenious of the methods. It uses the actual attribute as a pool of points which three levels of actions can use. A character can chose to do multiple actions directly based on the REA stat size - 3REA for short actions (reactions like defense), 5REA for medium actions (attack & spells), and 8REA for long (run away). In addition, skills and powers can increase or decrease the size of REA - like having a level 3 expert skill lets you take a 4 REA attack using that skill once per round. In the revised edition, you can even spend a few points to wait or delay actions. I just think of this as a direct attribute pool method.
Both d100 and JAGS require the character have available points in order to do defense. So that spending all your points on offense will leave your character unable to defend actively.
For me, it's important that quick characters actually act quick - and not just act first.