Conversion of Graphics to Image usually requires a rasterization and unfortunately it is the case here. Image constructor internally calls Rasterize when the input argument is a Graphics object. The only exception is Graphics[Raster[..]], where the image data is extracted directly from Raster. You can use ImageSize option of Graphics in your render function to avoid ImageResize call:

render3[i_, size_] := Image[Graphics[oneline[#, 0.5, 200] & /@ i, ImageSize -> size]]

Matthew,

You should provide some sample data otherwise its hard to say without trying things out. One thing that I notice is that imageResize is slow and not needed because you can specify the resolution in Image:

This is much faster

AbsoluteTiming[
 Image[RandomReal[1, {500, 500, 3}], ImageSize -> {200, 200}]]

than

AbsoluteTiming[
 Image[RandomReal[1, {500, 500, 3}]] // ImageResize[#, {200, 200}] &]

But beyond that I can't duplicate what you are doing without data.

Regards,

Neil