Speed of pitches

I’m 11 and throw up to 57mph and throw a slider would that be considered fast

A lot of coaches—and more experienced pitchers—will read this and see trouble ahead. You really shouldn’t be messing around with the slider at your age, and the simple reason is that certain growth processes in your arm, shoulder, what have you, are a long way from being fully developed. The general advice is, and I agree with them, is that you should wait until you’re about sixteen, when those growth processes are pretty much complete, before attempting the slider—or any other breaking pitch like that.
What you should do is concentrate on your fast ball—four-seam, two-seam, whatever—and work on developing a good changeup. Most pitchers at your level don’t need any more than that. My personal recommendation is the palm ball (which I acquired at age 11)—it’s a nice pitch, easy to pick up, and easy to throw and control because you throw it with the same motion and arm speed as the fast ball. With these two pitches you should do all right until about age fourteen, when you might think about a curve ball. But the slider can wait; it’s not going to run away.
I speak from long experience. True, I had a curve ball at age 11, but it was something that came attached to my natural sidearm delivery, and because I worked at it and didn’t overuse it. I had no problems with it. I also picked up, in addition to the palm ball, a good knuckle-curve, and I didn’t overuse it either; one thing I knew was that because I didn’t have a fast ball to speak of and would never be an overpowering pitcher I went the other way and became a snake-jazzer—and a very good one. I didn’t worry about speed; what I was interested in was control, command and good sound mechanics, and you would do well to do likewise. The speed, if you have it, will come in due time. 8)

I agree…
Concentrate on developing velocity and if you want another pitch to learn I’ll advice you to look into a change-up. This pitch can be very effective (even more then your slider when thrown properly) when you also gain velocity.

well the way i throw my slider is just a different grip but i have the same arm motion as my fastball

You are still really young, but 57 mph for an 11 year old is pretty fast I’d say. I threw about 55 when i was 11 and noone could hit that. I’d learn to throw a changeup.