Thunderbolt 3 may only reach a maximum data transfer rate of around 7Gbps to 22Gbps even though it is advertised with 40Gbps.
Although Thunderbolt 3 is advertised with a bi-directional total data transfer rate of 40Gbps, simple data transfer like networking data or storage data are limited to a total of 22Gbps as per the official Thunderbolt 3 specifications.
Depending on what CPU, Chipset, Memory, Hard Disk Drive, and the amount of available PCIe Lanes used, the actual data transfer rate can be limited even further.
For information relating to Dell computers, see Thunderbolt 3 40gbps Data Transfer Rate.
More information can be found from Thunderbolt Technology.