Although the Thunderbolt™ 3 standard is capable of data transfer speeds up to 40Gbps, not all TB3 cables can support this bandwidth. For example, some cables over a certain length 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:
20 Gbps Thunderbolt 3 Cable
40 Gbps Thunderbolt 3 Cable