SharePoint going into bad performance (delay) only when users logged in.

Hello All,

My problem is happened when users login into the website, anonymous users working fine with perfect performance.

Infrastructure as below:

- SharePoint 2010 Server.

- one application server and 3 frontends.

- two sql cluster servers connected to EMC storage.

I tried NTLM & Kerberos . and also tried the sharepoint configuration wizard. but the problem still occurred

the website was working fine for more than 1 year, I tried to remember what did I change and I am sure there was nothing changed in the SharePoint environment.

do I have to check anything with the system admin or the network engineer? or there is configuration I should be aware ??

Thanks

Omar Attari