cartoon rendering.

"Toon rendering" is a very broad term for pretty much any non-photorealistic shading technique that attempts to simulate the look of hand-drawn cel animation. A toon renderer may (or may not) include a custom lighting model, a quantised colour palette, stylised specular highlights, rim lighting, a border outline, and many other aspects. But, none of these are specific to toon rendering - they're just applied in a manner to give a particular artistic effect.

So I don't think anyone can answer whether toon rendering in general is any more or less complex than, say, any other sort of rendering - it depends on the particular properties of the shaders in question, the rendering path used etc. If you want to measure the performance of a specific toon renderer compared to another specific renderer then look at the rendering statistics window in game view.