I have Synology DS718+, which I am testing with one user (myself) currently.
My transfer rates from my laptop to Synology of around 2 MB/s. However copying FROM Synology back to laptop is around 30 MB/s! - therefore I am ruling network issues out.
Synology is connected directly to a router (1000Mbps) with a cat6 cable. Also in the control panel the interface setting show 1000Mbps, full duplex. Laptop is connecting using WiFi, so I don't expect 1GB, but 2MB is a bit extreme.
My laptop is Ubuntu 18.04, interface set to 1000Mb.
SMB is enabled with SMB2 to SMB3. If I disable SMB Transport Encryption Mode I SOMETIMES get to whooping 3 MB/s
I am about 5m from the router, no walls etc. in the way.
As per Freddy's suggestion (thanks Freddy) I tried different file sizes - always looks similar. However I noticed the speed is increasing very slowly - this can be seen clearly when copying larger file, i.e. 3GB. The file started as always at 2MB and by the end of the copying process transfer rate was ~7.5 MB.
Transfer rates over FTP are exactly the same - very slow copying to Synology and 10-12x faster copying from.