Probably caused by browser add-ons, interactions with social networking sites, antivirus software running in the background, or in the worst-case scenario, malware on your computer. Spend some time to run an aggressive virus/malware check, and check your installed programs to see if you have some CPU cycle hog running without your knowledge. They're often bundled with games and other downloads. Browser toolbars sometimes cause delays as well. It could also be the network or ISP, or even the DNS server you use.
Good luck fixing the problem. It's a pain to solve, but it can be done.
Mozilla Firefox + Ad Block Plus = My G@G never froze. But the messaging page in G@G really sucks. It doesn't instantly notify new messages and my messages take a while to appear on the thread after I have clicked send.