Servo speed is stated in seconds. Just as a car has a 0-60 mph time, a servo has a 0-60 degree time. The lower the 60 degree time the faster the servo operates.
Servo power is stated as ounce-inches (oz-in.) This is the maximum amount of power the given servo will apply with a one inch arm. For example, if a servo has a power rating of 16 oz-in, the maximum amount of power it will be able to apply with a 1” arm will be 1 lbs.
If you are needing to find out how many pounds a servo will lift or push with a 1” arm, simply divide the oz-in. number by 16.
oz-in. / 16 = pounds of force (1” arm)
If you are planning on using a 2” arm, you will then need to divide the pounds of force once again by 2.
An HS-425BB has 57 oz-in. of power
57 divided by 16 = 3.56
3.56 would be the pounds of force with a 1” arm
If you were to put on a 2” arm on the HS-425BB:
3.56 / 2 = 1.78
1.78 would be the pounds of force with a 2” arm
If you were to put on a 3” arm on the HS-425BB:
3.56 / 3 = 1.18
1.18 would be the pounds of force with a 3” arm