Some USB-C controller cards do not support the reversibility of the USB-C connector, which means that some cable and host controller combinations only receive USB 3.1 speeds in one orientation. To determine if this is the cause of your issue, unplug your USB-C cable and flip the cable upside down and plug it back into the USB controller card.
Although USB-C supports transfer speeds up to 10 Gbps, all of the devices in your setup must support USB 3.1 Gen 2 (10 Gbps) in order to transfer data at that speed. The maximum data transfer speed you will experience is the fastest transfer speed that the slowest device in your setup supports.