Ultimately I want to be able to control up to 8 servos using node.js - I'm able to control the servos using either pi-blaster or pigpio, but how do I control their speed with which they move?
When using servos that rotate through 180o, one sets the desired position of the servo arm and the servo will try and "get there" as quickly as possible. There is no explicit control of rotational speed on a servo. If one wants to attempt to reduce the speed, one can ask the servo to move to a target position by repeatedly moving to sub fractions of that position and delaying between moves ... however this is unlikely to deliver a desirable result.
For servos that were designed for continuous rotation, some of these use the PWM signal as an indication of their rotational speed. If we assume that a PWM signal is between 1000 and 2000usecs then 1500 means stationery, 1000 means maximum speed in one direction and 2000 means maximum speed in the other direction. Other values between 1000 and 2000 will rotate in the corresponding direction but at less than maximum speeds.