Testing LAN network speed

If you want to test pure network speed, iperf is the tool to have (https://iperf.fr/iperf-download.php). But you have to install on both machine testing the link (client/server). I've seen a couple of ways to get this running on synology, one beeing to run a docker container with iperf (https://hub.docker.com/r/networkstatic/iperf3). Ideally always test with a wired connection to limit factors (wifi interference).

Here is a few steps i would do in order to find the problem and test network speed:

First, make sure that the links between your NAS and network is correctly beeing displayed as gigabit and that your cable are OK.

Then i would try to transfer files using another protocol. You didn't mention it but i'm guessing you are using SMB to transfer files, which is not the fastest transfer protocol out there. Sometimes Synology NAS are using conservative config like suporting SMBv1 and SMBv2 which are slower. Try to use FTP for example, which is usually faster. If it's faster using FTP, you can then try different SMB configuration to get the speed you should be getting.

Make sure your drive are OK in your NAS. Faulted drive in a RAID array can cause performance issue while still beeing functionnal.

Like @John mentionned, you can always connect to the box directly by using static ip, bypassing your whole network to make sure that's not the problem.