Standard USB-C Data Transfer Rate

The Apple USB-C charging cable is only capable of USB2 speeds at best. If you want to know the maximum transfer rate of your USB-C charging cable, then measure the speed by reading data from a USB3 SSD. Almost any SSD will be more than fast enough for the test of USB2 speeds (at least when reading from an SSD) as the maximum USB2 speed is about 35-40MB/s. Reading from an SSD will give you the fastest speeds since writing to an SSD is usually slower (sometimes much much slower with low end SSDs).

Those systems are sold as chargers and the rating is in watts not Mbps or something similar.

I'd imagine their data transfer rate to be rather slow as they are not designed to optimally carry data. You can use the charging cord for a wireless keyboard to make it a USB type in function, but the data rate is still slow. Doesn't need to be fast.

If you need fast transfer speeds, don't expect one of these cords to do that.

Apple does not specify whether the cable is strictly a charging cable or what USB standards it supports.