The speed flow between 2 devices always depends on the performance of all devices the flow goes through. For example, a mac-mini 6,2 with i7 CPU + 16Gb ram provides a nice 980Mbps iperf speed in Download direction, but I barely get 250Mbps in upload direction. After looking for the reason, I figured out the onboard network chip had most buffers assigned to the RX, and had only few buffers on the Tx side forcing the CPU to interrupt other work to handle the packets to bt sent through the device.
Whatever happens - check all systems that are connected.
Since I know these things (some years now), when I buy a new device I make all sort of tests in the beginning to confirm the speed & performance is right. If it is not right, I send it back with a full report.
Never had a problem with that 
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