How many times a second should CADisplayLink's displayLink be called?

Question

I have a CADisplayLink running in line with Chipmunk Physics, and I'm getting very slow performance. I put an NSLog in the method that's called on the CADisplayLink update, and it's being called an average of 22 times per second. I was under the impression that that should be nearer 60. I have the frameInterval set to 1, so should it be 60fps, in a perfect world? The delta times are averaging around 0.0167 seconds (and 1 / 60 IS 0.0167, which is confusing me even further).

I just have four walls around the bounds of my screen and just eight circle-shaped bodies on-screen, updating to UIButton instances on each call, so I don't think I'm doing anything that should tax it to this extent on both my 4S and iPad3. I'm applying a random force to each button once every 2.5 seconds in a separate method. Running in the simulator is butter-smooth, so it's a device-only issue. Can anyone help me spot what's causing the slowdown here, and what I can do about it?

Here's the relevant code, first that which sets up the link:

[NSTimer scheduledTimerWithTimeInterval: 2.5f target: self selector: @selector(updateForces) userInfo: nil repeats: YES];
_displayLink = [CADisplayLink displayLinkWithTarget: self selector: @selector(update)];
_displayLink.frameInterval = 1;
[_displayLink addToRunLoop: [NSRunLoop mainRunLoop] forMode: NSRunLoopCommonModes];

Here's the method that should be called (I think!) 60 times per second, but is called only 22 or so:

if (!gameIsPaused) {
    cpFloat dt = _displayLink.duration * _displayLink.frameInterval;
    cpSpaceStep([[AGChipmunkSpace sharedInstance] space], dt);

    for (LCBall *i in balls) {
        cpVect pos1 = cpBodyGetPos(i.body);
        CGAffineTransform trans1 = CGAffineTransformMakeTranslation(pos1.x, pos1.y);
        CGAffineTransform rot1 = CGAffineTransformMakeRotation(cpBodyGetAngle(i.body));
        i.button.transform = CGAffineTransformConcat(rot1, trans1);
    }
}

And finally, here's the method that's called every 2.5 seconds, to apply the random forces (updateForces):

if (!gameIsPaused) {
    for (LCBall *i in balls) {
        int randomAngle = arc4random() % 360;
        CGPoint point1 = [self getVectorFromAngle: randomAngle AndMagnitude: (arc4random() % 40) + ((arc4random() % 20) + 15)];
        i.body -> f = cpv(point1.x, point1.y);
    }
}

(Also, here's my method to get a vector from an angle, which I doubt is causing the issue):

angle = (angle / 180.0) * M_PI;
float x = magnitude * cos(angle);
float y = magnitude * sin(angle);
CGPoint point = CGPointMake(x, y);
return point;

Answer 1

Turns out I had a method on a different UIViewController in my storyboard that was firing every 0.1 seconds that hadn't turned off, and combined with the physics processing, was bogging things down.