Default [Q] SD card. How fast is a waste?

How fast is the OG Epic's SD card interface?

I am planning to give Link2SD a try. I will buy another SDXC card for this purpose. I do not want the SD card to be the speed bottleneck. But there is no point spending the extra $$$ on wasted SD card speed. How fast is the actual SD card interface on this phone?

Probably this has been discussed before, but I do not see it in the specs.

Thanks in advance.