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How much more taxing are UV's?

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If I was to switch over to UV mapping and have each face mapped how much more demanding is that on system resources?

Right now on screen I'll have upwards of a million polygons and I use vertex colors.

Is there a ratio?  maybe for the amount of time it takes to the shader to render 10 vertex color polygons the shader can color 3 polygons referencing a UV map?

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 Well I don't have exact # for you, but I switched a project from using dense vertex coloring to using texturing(implicit UV's though).

The texture based version was at least 25% slower, but much more detailed since it had unique color per pixel.

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A million polys, is maybe 400k verts. Colors are usually either 4 or 12 bytes per vert (1.5MB or 4.5MB per frame) , and uvs are usually 8 bytes per vert (3MB per frame). If you use a single 1024px texture for everything, and all of it is used including mips, and the texture cache works perfectly, then that's around another 5MB per frame, for a total of about 8MB per frame... 

A NVidia GPU from 10 years ago can do about 100GB/s, which at 60HZ is a budget of around 1.6GB of scene data per frame. You're about 200x under that budget :)

Another way to look at it is if that GPU can do 100GB/s, and you have 8MB of scene data, you'll spend at least 78 microseconds per frame on memory transfers.... That's pretty good. 

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