Is updating 1 character in a 200 character Text object more expensive than a 10 character Object

Questions & Answers
1

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A. Does Unity Text cache the pixels being printed to screen and only update they that change?

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B. Does Unity Text rewrite all pixels in the Text every time it gets set?

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2

Font data is cached as textures - yes. UnityEngine.UI.Text component’s “pixels” are not cached anywhere. Any string you wish to display is converted into a stream of vertex data (vertex pos,uv coords etc.), more commonly known as a “mesh”, and sent to GPU for rendering.

So

Is updating 1 character in a 200 character Text object more expensive than a 10 character Object

no. It doesn’t matter (for GPU). Just make sure you’re not re-creating this string every frame (on CPU-side) because that would make a small difference indeed (GC pressure).