After all, it is much easier to volley a ball that comes with pace than one where you have to create your own pace. Right?
In very general terms - faster shots are easier to volley IF YOU PRACTICE THIS or are used to seeing fast shots at net. For those who are not used to the pace at net, then I think it's harder at first. Do enough fast paced net balls, and one adjusts to make smaller motions and much less racket head movement.
So it depends. Slow balls at net do reveal bad technique - no question about that.