Some of the comments I have about the speed of pulling revolve around the level of group members to the Gaurdian.
If I as a guardian am of higher level I pull faster, confident in the pull because I know the con color. When group members are higher, Im not always sure of the pull because we are usally in a tougher area and mobs are orange and red with up arrows to me, I pull slower.
Secondly, unfamilarity with an area will slow me down a lot. I dont know where all the mobs are hiding.
Third, how I pull either with a taunt, by using a bow, or just hitting the creature. I tend to use a bow if in unfamilar areas or im uncomfortable with the group or im just starting out with a group. I'll taunt with a hit if the mob is easy for me to kill and I will use a taunt when im in a groove or im familar with an area.
All of these affect the speed at which I pull, but Im also cautious by nature which tends to slow me down.