What is the USB storage transfer speed for Archer C7?

I just bought this router and the USB storage speed is extremely slow, about 5 MB / s. This is 7 times slower than the USB 2.0 transfer speed that I get on PC with the same drives (4 TB Seagate Expansion USD 3.0 and 3 TB WD Passport USB 2.0).

So, is this 5 MB/s the speed at which the storage is supposed to run? Because this is really not very useful if any transfer would need e.g. 7 hours instead of 1 hour.

The firmware is latest version. I'm considering to return the router while I can. I read about older TP-link models runing at about 10-12 mb/s, why is Archer C7 so slow?