What kind of drive are you using?
Ready to use portable drive or you retrofit a SSD into housing you bought?
Which model of iMac?

Of course Thunderbolt are miles better, USB will only give you burst speed. Thunderbolt are consistent. They exist and constantly getting revision for a reason.

https://www.facebook.com/maccores/photos/a....495240730545165

You did not answer my first few questions.

Try get a better quality casing.
And if you boot and benchmark from the same drive the speed will definitely suffer.
Also read about USB tech, they are only good on burst speed.
Want to boot and do some short troubleshooting with USB maybe still ok, if long term working over USB it is not going to cut it.

On a side note, your unit should have USB-C, that should be better over the USB3.

This post has been edited by wei: Sep 9 2020, 11:07 AM

Try put it into any internal SATA slot and see it behave the same. If same, it probably have a busted controller chip. Provided you have done re-partition and reformat.