USB-C external drive is very slow on iMac Pro but very fast on Macbook Pro

Hmm... that is strange.

Here are some questions to ponder over.

What year model is the iMac or iMac Pro in the office?

Identify your iMac model - Apple Support

What other USB drives/devices are connected to the office Mac?

(those can compete for overall USB band width speed)

Does the office Mac have any AV-ware installed on it or use a VPN?

(either of those can cause general slowness and other issues)

Is the office Mac starting up from the internal drive or a network drive?

(starting up from or moving data to a network drive is really slow)

The 2017 iMac Pro has 4 Thunderbolt/USB-C ports, yes/no?

1) Try plugging the external drive directly into one of the iMac Pro Thunderbolt/USB-C port and not the hub.

2) If the hub and display are plugged into port 1 or 2, then try plugging the external drive into the 3 or 4 port.

3) Basically you want to stagger the load of devices, some on the 1 and 2 ports and some on the 3 and 4 ports.

Try with cheapy monitor and hub disconnected? Also LAN cable (two tests)

Try moving the silver hub and monitor cable over to the 3rd or the last open port.