Once the switch is enabled, run the app and look for any images
that are outlined with a flickering box. The box flickers from
frame to frame if a new frame is being rendered.  For example,
perhaps you have a group of objects with opacities that are rendered
using saveLayer. In this case, it’s probably more performant to
apply an opacity to each individual widget, rather than a parent
widget higher up in the widget tree. The same goes for
other potentially expensive operations, such as clipping or shadows.

Note:
  Opacity, clipping, and shadows are not, in themselves,
  a bad idea. However, applying them to the top of the
  widget tree might cause extra calls to saveLayer,
  and needless processing.

When you encounter calls to saveLayer,
ask yourself these questions:

Does the app need this effect?

Can any of these calls be eliminated?

Can I apply the same effect to an individual element instead of a group?