The response of SSR is much better than standard servos. Thats what you are paying for. That response time is good at either 6v or 7.2v. The speed and power of the servo will of course be lower at 6v but you are still getting the benefits of SSR. (as long as you've enabled FH3/FH4T and SSR modes on the radio which should be either the M11X, M12 or Exzes X or Z)

You could also consider an ESC that puts out 7.2v such as the Tekin RSX for example. Alternatively an external BEC or rx pack. In my opinion the servo is probably plenty good enough at 6v anyway.