Network latency problem

We have a multiple computer environment with apps 15 workstations all running Windows 7. Starting about a week ago we started to experience network latency. All workstations are running Symantec virus protection and spyware protection and there does not appear to be a virus Not every workstation is experiencing latency. At least half of the workstations are exhibiting issues.
We have a Sonicwall Firewall as our main router.

Both internal applications and well as external websites are experiencing latency whenever they are going out to access non-local resources and network printing is very slow.

We have looked for Trojans and viruses and are not seeing anything, we need help.

The servers are not experiencing latency, just the workstations.

We installed a new Netgear smart switch last night in hopes this would fix our problem. It did not. But it did do this - we activated the intrusion protection on the smart switch. Now it constantly deactivates the internet port.
Anyone help?

 

Welcome to majorgeeks!

Is it the same workstations that consistently have issues? Are those stations related - i.e. ordered as identical from a vendor? Is there something in their settings that is the same or the way they are used (all sales people) or is there an application that most but not all users run? Do those users work in an adminstrator account regularly where the others do not?

If there is one slow running system that you can use as the test, i would roll it back with system restore. There could have been any kind of update to windows, drivers or software that is causing this. That the problem is far reaching, updates would be my first guess.