使用场景：稳定的视图（layer）被反复使用或进行动画；
本质：牺牲内存解放cpu；
反例：牺牲内存牺牲cpu。
单次使用或者视图有变动，shouldRasterize不会有任何用途，反而会牺牲内存。
shouldRasterize会触发离屏渲染，所以会降低一次性渲染的效率。
Bottom line if you have a complex view (i.e. relatively expensive to re-render) that you are animating, but for which the animated view is not itself changing, rasterizing the layer can improve the performance by not re-rendering the layer all the time. But it does this at the cost of memory (saving a rasterized image in memory).
But, if you animate a change within the layer, the shouldRasterize can adversely affect performance (because it's going to re-rasterize the layer for each frame of the animation).
Generally, if animating a complex set of layers that, themselves, are not changing, then you can set shouldRasterize to YES, do the animation, and then turn off shouldRasterize.
https://stackoverflow.com/questions/19405741/when-should-i-set-layer-shouldrasterize-to-yes
Would setting shouldRasterize to YES improve animation performance though? – Senior Sep 12 '12 at 15:47
作者：Heikki_
链接：https://www.jianshu.com/p/d56ba5b7bc27
來源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。
而光栅化会导致离屏渲染，影响图像性能，那么光栅化是否有助于优化性能，就取决于光栅化创建的位图缓存是否被有效复用，而减少渲染的频度。可以使用Instruments进行检测：
当你使用光栅化时，你可以开启“Color Hits Green and Misses Red”来检查该场景下光栅化操作是否是一个好的选择。
如果光栅化的图层是绿色，就表示这些缓存被复用；如果是红色就表示缓存会被重复创建，这就表示该处存在性能问题了。
作者：路漫漫其修远兮Wzt
链接：https://www.jianshu.com/p/ce73e3836730
來源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。
UIView, one of the animation options is to allow rendered content of the views its animating. This is off by default, which means the animation will take a snapshot of the view and animate that. So in that case rendering would not really be affected. If you're using CAAnimations, you will get a performance increase by settingshouldRasterize, because the animation will only animated the rasterized layer, and not ask it to draw each frame of the animation loop. – gdavis Sep 12 '12 at 17:58