No, this is not off-topic.
This only demonstrates how a specific device vendor deals with ovbious problems. They try to ignore as long as they can and blame everyone else around instead of their own product (see what the 'Principal Software Engineer at Raspberry Pi (Trading) Ltd' tries to point out: 'TBH, Microchip should be sorting this sort of thing out - it's their chip, firmware and driver'. They chose a new USB/Ethernet chip, they never tested this thing obviously and now they blame the chip vendor. And at the same time they delete every post in their forum that shows them the problem -- crappy performance in RX direction with tons of retransmits -- and they not even follow most basic troubleshooting advise. Are you f*cking kidding me?)
Given that the Raspberry Pi platform suffers from weird USB problems since day one it's really inacceptable that they blame the chip they attach to their somewhat special USB implementation. And then there's always the closed source problem. The main operating system on these boards is an RTOS called ThreadX for which no source access is available. If the problem has to be fixed there... no one outside the mythical Foundation or RPi Trading can do anything (RPi was never and will never be Open Source Hardware, it's the most closed SBC platform I ever dealt with).
Given that RPi Trading's only mission is to sell mediocre hardware to clueless people there are not that many incentives to make the RPi world an open place. Keeping their users clueless (censorship, banning critical voices) is just a logical consequence.
Another user took over and tried to raise basic awareness for the problem they try to ignore so hard: https://www.raspberrypi.org/fo…e134e86&start=75#p1294035 (archived version here in case they again do what they do all the time: censor everything that doesn't fit into the 'RPi is great' illusion).
242 Mbits/sec in RX direction and still retransmits is still a massive indication for a very basic problem (if you don't get at least 310 Mbits/sec with USB2 attached Gigabit Ethernet there's something seriously wrong!). And if @dogg!~ is right and his TimeMachine problems even occur with Ethernet set to 100 Mbits/sec on the RPi 3 B+ while everything works fine with 3 B then they should start to focus on their weird USB implementation and integration with Microchip LAN7515 (prior to that it was LAN9514 -- comparison).
But suffering from a typical Dunning-Kruger syndrome problem (enabling people on RPi Trading's payroll to censor expecting allegiance for their mediocre hardware) this will take ages and prevent smart people helping in the process. And even if they can solve their very own USB problems another time it still changes nothing and any Raspberry Pi as NAS is worst SBC choice possible. There is only one single USB2 connection to the host which slows down everything as hell. Gigabit Ethernet in such a situation was already known to become a problem.