Well I see three items.
1: if you just added a new drive, drive balancer may be hogging your cpu power for a few days.
2:you may have issues with a 10/100 router(prior or now)
3: you may have a bad component in your network.(cable,nic card,wifi dongle.)

_________________
VGA/PS2 Keyboard/PS2 Mouse/Serial debug board
Popcorn C-200
Popcorn A-200
Dune HD Connect
Rasberry PI OSMC(KODI)
ReadyNAS RN316
Amazon FireStick
Amazon ECHO
AnyDvD
MyMovies

Extremely Slow? What would you consider slow.
I have a Giga network and can transfer a 1.5 gigabyte movie file in about 30 seconds, haven't timed it, just guessing, it is pretty fast.
Now if you are moving 1 - 2 - 3 TB of data, then even on a giga network that is going to take hours, on a 10/100 that could be Extremely Slow.

So I guess what I'm saying is it may depend on what you are doing. And as described above, your network is only a fast as the worse part. Wire/switch/nic cards/is the server clean, meaning not infected?

Good luck and if you solve the problem, let us know, it could help others.

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[X510 CPU/RAM/All 2 TB Red's] [X510 CPU/Sync'ed Backup/All 2 TB Hitachi's]
[X510 /2 in box]
[X710 LIAN LI DIY W2016E] Stablebit Drivepool [EX-503 LIAN LI 5-bay USB3 External]
Fear the Cloud

weird.
The original poster is gone just our replies appear now.

_________________
VGA/PS2 Keyboard/PS2 Mouse/Serial debug board
Popcorn C-200
Popcorn A-200
Dune HD Connect
Rasberry PI OSMC(KODI)
ReadyNAS RN316
Amazon FireStick
Amazon ECHO
AnyDvD
MyMovies