It depends on how much bandwidth each is using. There is a set amount of data each one can transfer, however the interface between the motherboard and ports is less than the sum of the max data transfer of the ports. As a result, when transferring more data than the motherboard interface, the data transfer rate in lowered so as not to exceed the interface bandwidth limit. So yes, one, or both of the ports, can be bottle necked if they are both transferring data at the same time (as almost max saturation).