It's probably expected, Thief does use quite a bit of processing power and the game isn't that optimized yet. Changing the video settings wont necessarily change the load on the processor by a great deal however.
Depending on the care you take, keeping dust out of it, watching temperatures etc a laptop can last for at least 4-6 years without a decrease in performace. The usual time it takes for pc components to degrade is something like 5 years.