Could be a bad cable, one dodgy wire in an ethernet cable will drop you to 100mbit link speed, or it could be that your adaptor isnt actually gigabit, or the port you are plugging into isn’t gigabit.
I’ve used a USB3 Ethernet adaptor on a Samsung Note9 and a Fold3 and both supported the full gigabit sync with the expected speeds for a cheap usb ethernet controller.