a computer with a USB3.0 drive plugged into USB2.0-boot compatibility is the lowest.
This is not true. A USB 3 drive has two sets of contacts, one for USB 3 and one set for USB 2. if you connect a USB 3 drive to a USB 2 port, then the USB 2 contacts are used and the drive will work as a USB 2 drive.
If you connect a USB 3 drive to a USB 3 port using a USB 2 cable, then it will also appear as a USB 2 drive.
The usual problem on old PCs with USB 2 ports, is that the BIOS does not recognise devices which report their total size above a certain size - e.g. 4GB or 8GB or 16GB. This is why some people think that the problem is USB 3 when in fact the problem is to do with the size reported by the drive.
Of course, the other issue is the contents of the USB drive. To do any meaningful tests, you should use dd or an image program to copy the sectors of one drive to another drive. Then they will have identical contents and you can do a meaningful test.