Cocoa drawing on different screens loses performance

I have a document-based app, where each document has one window with an NSScrollView that does some (fairly continuous) drawing using only Cocoa.

To call the drawing, I am using a CVDisplayLink, outlined in the code below:

- (void)windowControllerDidLoadNib:(NSWindowController *) aController {
     //other stuff...
     [self prepareDisplayLink]; //For some reason putting this in awakeFromNib crashes
}

//Prep the display link.
- (void)prepareDisplayLink {
    CVDisplayLinkCreateWithActiveCGDisplays(&displayLink);
    CVDisplayLinkSetCurrentCGDisplay(displayLink, ((CGDirectDisplayID)[[[[[self windowForSheet]screen]deviceDescription]objectForKey:@"NSScreenNumber"]intValue]));
    CVDisplayLinkSetOutputCallback(displayLink, &MyDisplayLinkCallback, self);
}

//Callback to draw frame
static CVReturn MyDisplayLinkCallback(CVDisplayLinkRef displayLink, const CVTimeStamp* now, const CVTimeStamp* outputTime, CVOptionFlags flagsIn, CVOptionFlags* flagsOut, void* displayLinkContext)
{
    NSAutoreleasePool *pool =[[NSAutoreleasePool alloc]init];
    CVReturn result = [(ScrollView*)displayLinkContext getFrameForTime:outputTime];
    [pool drain];
    return result;
}

//Drawing function:
- (CVReturn)getFrameForTime:(const CVTimeStamp*)outputTime
{
    [scrollView lockFocusIfCanDraw];
    [self addToCurrentPostion:(dist/time)*CVDisplayLinkGetActualOutputVideoRefreshPeriod(displayLink)]; //Redraws the scrollview];
    [scrollView unlockFocus];
    return kCVReturnSuccess;

}

//Set the display when the window moves:
- (void)windowDidMove:(NSNotification *)notification {
     if ([notification object] == [self windowForSheet]) {
         CVDisplayLinkSetCurrentCGDisplay(displayLink, ((CGDirectDisplayID)[[[[[self windowForSheet]screen]deviceDescription]objectForKey:@"NSScreenNumber"]intValue]));
     }
}

//Start or stop the animation:
- (IBAction)toggleAnim:(id)sender {
     if (CVDisplayLinkIsRunning(displayLink)) {
        CVDisplayLinkStop(di