Ok we all know obviously that it's all Hades' fault and part of his plot to triuph and take over the world.. Yes! It is HADES who causes your ( Hero ) PC to lag, to stop you from saving the world ( again ). Heck it's Hades' main power and ability that he uses against the Hero is to make his perception of reality faulty... causing, what we call simply: LAG. Okay well I have one solution that might help you combat his strong psychic powers and render his strong ability less powerful.

Okay jokes aside, Here're the instructions:

In your Documents/My Games/Titan Quest - Immortal Throne/settings folder there is a file called "options.txt" open it.
Scroll down and find a line
forceSingleProcessorMode = false <--- Make sure it's false! If it says true, change it to false. This should decrease Lags by a Large margin if you have a multi-core processor.

Hope it helps! If it does would be neat to get it a stickie. Also some people that actually found this of help could actually post here and say so, would also encourage and prove to others that it works.

Thanks!