On occasion I've seen comments online along the lines of "make sure you set 'bs=' because the default value will take too long," and my own extremely-unscientific experiences of, "well that seemed to take longer than that other time last week" seem to bear that out. So whenever I use 'dd' (typically in the 1-2GB range) I make sure to specify the bytes parameter. About half the time I use the value specified in whatever online guide I'm copying from; the rest of the time I'll pick some number that makes sense from the 'fdisk -l' listing for what I assume is the slower media (e.g. the SD card I'm writing to).
For a given situation (media type, bus sizes, or whatever else matters), is there a way to determine a "best" value? Is it easy to determine? If not, is there an easy way to get 90-95% of the way there? Or is "just pick something bigger than 512" even the correct answer?
I've thought of trying the experiment myself, but (in addition to being a lot of work) I'm not sure what factors impact the answer, so I don't know how to design a good experiment.