I have an Ubuntu 16 NFS server running, I recently switched network cards in it, until that switch it was fine.
At random times it will get super slow browsing NFS, randomly. The only way to fix it is to SSH into the server and run
sudo service networking restart then it works normal again for awhile.
This didn't start until I switched to the secondary NIC card as the primary network card.
I also have Apache and PHP running on this machine, as well as an FTP server. I've tested it when it gets slow and neither Apache nor FTP slow's down during this time when NFS slows down. I can still upload at Gigabit speeds over my network over FTP, but not NFS until I restart the network.
So I know it's not the card, it's something in the NFS configuration, but I can't find where or what happened.