kozm wrote:1 forget the 220k, if the RPis are communicating with each other over the internet (the desktop pi with the NAS pi), 90+% bandwidth decrease happens. In LAN everything is fine.
2 i dont know
3 i dont know
Let me point something out before I start: it is very important to give accurate and relative information when asking for help as it give a better chance to get to the right answer.
The answers to 2&3 play an important role if the answer to 1 is that both systems are 220Km apart. First thing is (see 2) that 220Km can be the direct distance, but the "electrical" distance will be much higher depending on how the cables are routed. The answer to 3 is about the maximum data transmission speed that can be reached over fiber connections, fiber is the material used provide a higher bandwidth between communication devices.
On a distance of 220Km there will be multiple components (routers) and cables involved, each router will store data packets before re-transmitting the packet, each cable length involved adds travel time.
The total time for a packet to arrive at the destination and for the reply to be received can be called latency. You can find out how time gets lost on the network by comparing the ping response time, and the number of routers can be found with tracert.
The above is only the beginning, there is more. The next point is to look at how data is transmitted and how to information is "protected". I leave this as an exercise for you to identify what other layers might be influencing the performance, you can use the following keywords for this purpose: UDP, TCP, sshfs.
What you must look for is how the higher level protocols ensure that no packets are lost and packets are received in the correct sequence, and how latency will reduce the measured transmission speed.