Why can the speed fluctuate when copying to a Synology NAS?

Question

NAS

When copying large files from a PC to a Synology NAS located on the same local network, the speed fluctuates between 35-50MB / s via cable. In parallel, the torrents client does not work, nothing is downloaded. On the router, the CPU usage is in the range of 20-30%. What could be the reason for such uneven speed?

Answer 1

Standard TCP behavior when dropping packets. The synology cache overflows, it drops another TCP packet with data, the protocol considers that there is a congestion, and resets the data transfer window by half (it works like that), then it grows again until the cache on the receiver is clogged again.