Only the people who voted to close your question can answer why they voted to close it. In this case, five (5) people with a large aggregate of reputation points were among those voters. You may disagree with their opinions (and that's all they are really), but I feel you should accept the fact that good intentions do not guarantee an outcome you favor. And so I cannot explain why your question was judged as off-topic, but you may wish to consider the following:
There have been 33,973 questions asked on RPi SE. This figure includes 8,851 questions with no upvoted or accepted answers. Many of these questions should have been closed. Closing questions is absolutely essential to the viability of S.E. - it's a signal-to-noise issue.
There are many, many articles/blog posts/FAQs/etc on the Internet REF1, REF2 that address your question - or something similar. Here's one that you may wish to peruse.
It's fairly well-known that RPi have traditionally suffered from Ethernet performance shortcomings due to their decision to use the USB 2.0 pipeline to move the data. "The Foundation" copped to this years ago.. One might consider their pronouncements re Gigabit Ethernet as false advertising. Please also understand that Raspberry Pi is not an open system, and wrt the hardware and firmware, there is only limited visibility.
Again, this is no answer to your question, but hopefully some food for thought.