Are you checking on your server, for example monitoring backup status, are you backing up your server?
Are your server drives full? It could be causing that slow network performance.
Are there any other hardware devices failing on your server?
Let us review your server and repair the bad situation. Through out our years of supporting businesses we have come across many scenarios where the network was running slow, repaired bad hardware or have had to replace a server. We are available 24/7 to assist service your server.
Note: If you are running Server 2000 or 2003, today is the day to update they are no longer supported by Microsoft.