I did a search and found several "slow ethernet" threads but none of them matched my experience exactly so I thought creating a new one made sense. I'm running 10.12.3 (more specs below) and this is a periodic problem. I first noticed this when copying files to and from my Synology DiskStation and I thought it was an issue specific to that NAS but recently I noticed it affecting my general network performance as well. The network speeds in my examples will be NAS related as I have not had...
Periodic severe slowdown of network speeds
Periodic severe slowdown of network speeds