Although the Thunderbolt™ 3 standard is capable of data transfer speeds up to 40Gbps, not every cable can support this bandwidth. For example, cables over a certain length or made with certain materials are only capable of data transfer speeds up to 20Gbps.
To confirm the capability of your Thunderbolt 3 cable, refer to the information provided by the manufacturer. StarTech.com lists the capabilities of the Thunderbolt 3 cables on the individual product pages.
There are two different Thunderbolt 3 cable symbols: the 20Gbps symbol and the 40Gbps symbol. The 20Gbps symbol is composed of the Thunderbolt 3 symbol, and the 40Gbps symbol is composed of the Thunderbolt 3 symbol and the number three. These images are shown below.