Pretty quick, far faster than inter-process memory copy. The only way to be sure would be to set it up and to measure it, but on a 486/33 I could do this ~200K per second, on modern systems it should be a lot faster than that, more so if the processe(s) do not use FP. But I never actually tried setting up say a /dev/null implementation that used this, it would be an interesting experiment.