Performance question

Hello all. I was wondering. What kind of performance reduction is there in including files or using the auto prepended one for a file less than, say, 10 line? Specifically, I would think that the file IO would be a detriment to such a small file. Without the includes, code would be repeated, but in PHP, IMHO, speed is a much more important factor than code size. Anyways, what would be the performance factors in deciding what goes in an include vs inline code? ---Matt