USB transfer speed depends on many things.
the usb drive real read/write speed capacity (even a 3.1 certified usb key with bad quality/bad brands could have performance of a usb 2.0.)
The usb version on the computer you are plugged on.
The drive speed on this same local computer.
You should try to connect the computer to your local network and browse it trough windows sharing (smb).
By default, if you plug your batocera device to the local network ip will share its drives and you can browse it with “\\<ipaddressofbatocera>” and then copy your roms to the correct local folder.
Maybe it should be faster.
