I hadn’t looked into the file transfer speed of my ageing Synology DS413 getting slow. It turned out that my Windows 10 client was using a policy of requesting encrypted SMB.
The old CPU in the NAS couldn’t keep up with this and it really slowed transfers down. As encryption isn’t really an issue in a small home setup I decided to disable it on the NAS and the speeds immediately almost doubled. You can find the setting under Control Panel, File services, SMB, Advanced Settings. Set Transport encryption mode to disabled.
It may be necessary to change something on the Windows side should Microsoft require encrypted SMB in a future build but it’s working fine in the 1703 version.