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Using something like the Apple-provided CGLayer, you can draw the image once, then use it like a bitmap while it is in memory, AND they can keep it cached on the graphics card. These guys could easily produce code that does that.

Reference: https://developer.apple.com/library/mac/#documentation/graph... (it works on iOS too)

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In practice, CGLayer is not some pixie dust that magically makes your drawing fast. It helps only in a specific instance: when you are drawing repeated instances of the same content, into the same context. Otherwise, it ends up being extra complication for no benefit.

(Note that CGLayer is not related to CALayer or CGTransparencyLayer -- they are absolutely separate things. CALayers are incredibly useful in practice; CGLayer was kind of a dead end.)

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