I’m trying to copy the contents of my home folder to USB 3.0 and the transfer speeds are hopeless!! It’s starts around 100MB/sec and at around 1GB it drops to 5MB/sec??? Total size of the home folder is 274.4 GB and the estimated time 38+ hours!!!
Then your dirty cache on the RAM must be about 1G. The speed of 100MB/s is first the transfer speed to RAM and when it is filled you see the actual speed. It writes in the background. The RAM cache acts as buffer. So 5MB/s cannot be an USB3 Device, even if you plug in an USB2 device in an USB3 port, it doesn’t become an USB3 device.
The only problem I see here is that the buffer is way to high for your usb device.