I have a pretty underutilized Ethernet LAN at home -- basically I am usually the only one using it, everybody else is doing WiFi.

A strange thing that I have noticed is that I have inconsistent large file copy speed from my PC (Windows 10 Pro) to my NAS (Synology DS214+).

At times it will be around ~80-90MB/s and stay that way, but quite often it will be around 40MB/s and not raise above it.

Rarely it will also start at ~80 and drop to ~40 early during transfer.

I am usually copying from a SATA SDD, and I don't have any background (eg. torrent) transfers during the copy time.

The connection is: PC -> [gigabit switch ] -> [ router ] -> NAS

What have I checked:

  • Background apps/file transfer on PC -- practically never present
  • Other LAN traffic hoggers -- use WIFI, shouldn't affect Ethernet? Beside nobody is even streaming video or downloading anything during those times that I copy
  • Some tasks on Synology? Possible, but I generally have scheduled things to update during night

What other things I should check to eliminate issues?

1 Answer 1


This question comes up from time to time : This behavior is because what you're seeing is how fast do the caches fill-up on Windows and in the NAS.

As the disk is faster than the network, Windows will read from the disk at its speed and send the data over the network at a lesser speed. At this stage the speed you're seeing is basically that of the disk.

When the Windows memory cache fills up, the speed will drop to that of the network, since cache memory needs to be liberated in order to read more from the disk.

On the NAS side, if the network is faster than the disk, then you will see the same behavior, which may further slow down the transfer.

  • Thank you, but I am not sure this is the case. The theoretical thoroughput for 1 Gbps link would be around 125MB/sec. I am never seeing speeds higher than 90MB/sec, so, doesn't that mean I am seeing network speed always?
    – Gnudiff
    Jul 18, 2022 at 20:52

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