Maximum transmission speed G Drive 4TB Thunderbolt

What is the Maximum transmission speed G Drive 4TB with a Thunderbolt cable

I have a 500GB SSD (older model, silver case) and when I connect it with a Thunderbolt cable to my 2017 iMac I get a horrible 40+ MB/s. If I connect it to the USB 3.1 connector I get something over 200MB/s. I don’t quite understand why that is the case but it hampers my ability to use with my macbook air which only has USB-C Thunderbolt connections. I expected them to not be a bottleneck and have a question in to support on this issue.