USB-C SSD external drive extremely slow when connectet USB-C to Thunderbolt 3 via Apple original USB-C to USB-C charging cable

Well, that was exhausting but essentially on point.

I have an iMac (Retina 5K, 27-inch, 2017), 4,2 GHz Quad-Core Intel Core i7, 16 GB 2400 MHz DDR4, Radeon Pro 580 8 GB

My external SSD drive is a 1TB Adata SE800 SSD with USB-C connector.

This is what reads out when I connect it to one of USB-A slots in my iMac. And the workflow speed (premiere, photoshop, after effects) checks out.

However!

If I connect it via the Thunderbolt 3 / usb-c it's a whole different story.

Dreadfully slow. Is this not weird? Anyone has any idea what to do about this?

Thank you kindly for any insight.