Network Slowness and Monitoring

Is it only about wifi or everything is sporadically slow?

Everything slow unpredictably.

To external resources only, or to internal resources as well? You should be monitoring with a tool like Wireshark or PRTG.

For everything slow most likely cause is a problem with DNS.
Bring it into premises if not yet there.
Run namebench at times of sorrows.

no, dns is in premises and seems working fine.

Actually it would be good try to eliminate DNS. Could you set one work station to use Google's servers and see if things are far better? If not, then you need to check your network for congestion.

Eliminate DNS? how to handle internal traffic?

I mean eliminate DNS as being the issue. Well, trying to test with outside servers would just serve as a test. Could very well be something else, but mighy as well at least try it.

Use NetCrunch network monitor to visualize traffic inside your network (top talkers, slow ping response times, overloaded servers, other network parameters) and diagnose it this way. NetCrunch is paid software, but you can use it free for 30 days, install it, diagnose your issue and uninstall it if you don't want it. Lots of people use it for network diagnostics, since it can narrow down and create physical network maps down to per-port level on switches. I'm attaching a screenshot which might be of interest to you.

try google DNS 8.8.8.8 problem persist try removing NIC's and replacing them and or replacing network cable also