问使用phonegap的iphone应用程序运行缓慢

EN

@synthesize canvas, drawing; //Both UIImageViews
CGPoint touchPrev;
CGPoint touchLoc;

-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch * touch = [touches anyObject];
    touchPrev = [touch locationInView:self.view];
}

-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch * touch = [touches anyObject];
    touchLoc = [touch locationInView:self.view];

    UIGraphicsBeginImageContext(canvas.frame.size);
    [canvas.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSetLineWidth(context, 8);
    CGContextSetRGBStrokeColor(context, 0.8, 0, 0, 1);

    CGContextSetLineCap(context, kCGLineCapRound);

    CGContextBeginPath(UIGraphicsGetCurrentContext());
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), touchLoc.x, touchLoc.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), touchPrev.x, touchPrev.y);
    CGContextStrokePath(UIGraphicsGetCurrentContext());

    canvas.image = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    touchPrev = touchLoc;
}

-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    touchPrev = touchLoc;
}