Didn’t mean to disagree, odds on it is a bad cable - either faulty, or just border-line performance. Assuming that is, you have complete confidence in gigabit capability of whatever it is connected to at either end?
Maybe a more scientific approach than measuring obderved speed, which can be affected by other co-incidental factors, I would have expected there to be some way of interrogating the ethernet connection to see if gigabit had been negotiated. Using for example the GUI, telnet & CLI, whatever, to see whether it has negotiated 100Mbit or gigabit. Maybe some Busybox/Linux experts can suggest suitable CLI incantations...?
Alternatively, on some kit the flashing activity leds change colour when gigabit is negotiated. Maybe worth checking whether that applies to your kit, unless you already know.