Not able to get thunderbolt 3 speed

I have a 2017 iMac 27" and bought a nvm-e (Samsung 970e Pro) put it in a Sabrent enclosure, to use as an external drive. The speeds of the nvm-e are supposed to be in the order of 5GB/s. The TB3 is 40GB/s max, so I figured it would be blazing fast. NOPE, getting speeds of 4 -15MB/s

This is installing from 'Downloads' to 'Applications', and actually took 10 min or more. I am not sure why, the device is being identified as USB 2, when I go to my system report, I clearly have TB3

What is going on? Do I need to update a driver? OR is the Sabrent enclosure not work with the iMac? I tested the Sabrent on my Macbook Pro, was getting 800MB/s or better. So there is some issue with the iMac, just not clear what I have done wrong - OR what I need to do to correct this.

Appreciate any help, thank you.