iOS - Slow animation on iPhone 4S (but fine on simulator)

Question

I have subclassed a UILabel with the following code, which works fine - but any animations involving the subclass runs a lot slower than normal UILabels. I'm assuming Quartz is the culprit, but is there anything I could do to speed things up a bit?

- (void)drawTextInRect:(CGRect)rect
{
    CGSize shadowOffset = self.shadowOffset;
    UIColor *textColor = self.textColor;

    // Establish the Quartz 2D drawing destination:
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(context, 1);
    CGContextSetLineJoin(context, kCGLineJoinRound);

    // Draw the label’s outline:
    CGContextSetTextDrawingMode(context, kCGTextStroke);
    self.textColor = [UIColor whiteColor];
    [super drawTextInRect:rect];

    // Draw the label:
    CGContextSetTextDrawingMode(context, kCGTextFill);
    self.textColor = [UIColor textColor];
    self.shadowOffset = CGSizeMake(0, 0);
    [super drawTextInRect:rect];

    self.shadowOffset = shadowOffset;
}

Answer 1

What @MobileOverlord said is certainly applicable, especially the parts about about profiling.

I will note that setting shouldRasterize=YES is not a catch-all solution (why wouldn't Apple have set it on as default if that were the case?). Yes, it can improve scrolling performance, but it can do so at the expense of memory use since you can end up with a bunch of large images sitting around in cache.

It also incurs overhead at the time of creation, I believe (but would have to check to be sure) including an off-screen rendering pass to actually create the rasterized copy. Depending on how the layer is used, this could actually hurt performance.

An additional factor to consider is whether your view has any transparency. If you can guarantee to the frameworks that your view is opaque (cf. setOpaque/isOpaque), they can optimize rendering by not considering all the complexities associated with alpha channels, etc. Similar considerations apply to CALayer.

Finally, outside the block of code you showed, did you do anything sneaky to the backing layer (e.g. set a shadow or corner radius)? That's a quick way to kill performance on animation too.