Basics first, open the server manager and see if your RAM is being maxed out, or if your system drive is close to full.
Check if anything has been added to your network recently that could be broadcasting on the net (use Wireshark to check)
Check that your routers and switches are transmitting data at the correct speed, restart if necessary.
If the problem is not in your base architecture, look at what has changed recently, have you added any smart hotfolders or other automated processes?
Are you running RBA and have you added or changed any rules?
Have you got a rule that runs on a timer eg. for archiving jobs and have you had a large number of jobs all hit that timer at the same time?
We have a timer that exports jobs 2 weeks after they have been set to completed, if we have a few big jobs all completed at the same time, we'll see a brief slowdown 2 weeks later when those jobs go to export.