usn-c

My mobo is Asus Prime X570-P but it doesn't have usb-c, so I am trying to add it, however when I stick a usb-c card into 2nd pcie 16x slow, I think it becomes 1x slot since speed is capped at 200MB/s. I also have a thunderbolt 3 external hd with USB-C connector and I connected to the back of it...