Does having a SD-card in your camera slow down CF card write speed?

[Markus]

Have anyone tested if having a SD-card in the 5D MK III slows down CF write speeds? I guess it should not if you don't write to both but it would be nice to be certain. I would try it myself but it seems you need to load the software from SD in the latest versions.

http://www.canonwatch.com/why-you-should-better-not-use-an-sd-card-with-the-eos-5d-mark-iii/