The slow down from using real-space convolution rather than FFT is already a huge hit.  The extra factor from using Lanczos rather than Quintic, say, isn't as big.  So probably people should use whatever `x_interpolant` is appropriate for the meaning of their image, and if they care about the speed at all, they can experiment with the tradeoffs of using different choices of interpolant.

It's probably more useful to put a comment in saying that real-space convolution is really slow for InterpolatedImage in general, and describe my advice for how to get xValues with FFT convolution.  Probably this belongs in the doc for xValue rather than InterpolatedImage, since it will be true for many other profiles as well.