TL;DR: Maximum pipe speed, assuming both programs are written as optimally as possible, is approximately the speed of what one core in your system can read/write; this is because, essentially, the kernel maps the same physical memory page from one program's stdout to the other's stdin, thus making the operation a zerocopy (or a fast onecopy in slightly less optimal situations).

I've known this one for awhile, and it makes writing shell scripts that glue two (or more) things together with pipes to do extremely high performance operations both rewarding and hilarious. Certainly one of the most useful tools in the toolbox.