So the servo speed you list is a servo without counter torque. A servos speed slows as more force is applied against it.
As a mechanical engineer, I will tell you there are three reasons that they are slow:
A) servos are used that are too weak (because hexapods require a lot of servos, people get the cheap kind)
B) poor mechanical design - position of the servos, length of the legs, addition of springs, etc could really help
C) unoptimized programming
Mechanical design is something even a beginner can guess with fairly accurate intuition . . . but you can make your design significantly better if you plan it out and crunch the numbers. I spend weeks in CAD design, run stress analysis simulations, calculate torque and applied forces, determine expected losses, dynamics, etc . . . perhaps overdesign I dunno . . . in basic diff drive robots you do not need to do this, but for hexapods and robot arms its almost required to get your robot to work as planned.
By calculating forces, you will know if your solution will work/fail without ever building it.
And to motivate you to study up - picture spending a wasted $100 for a motor that turns out not to have the torque you expected. Calculating can save you money!
One day I'll write a tutorial for all of this . . . for now you have this:http://www.societyofrobots.com/mechanicsbasics.shtml