You'r right I don't spend time 20ms to wait but I spend between 1-2ms for pulses.
With 8 servos, this means I spend between 8-16ms for pulses every 20ms. So I still have between 55%-71% to execute my code and not 20% (if my operations are correct). It is not so bad. It was enough to control my robot with sensors.
(1000ms/(20ms+16ms)*20ms=555ms)
(1000ms/(20ms+8ms)*20ms=714ms)