That isn't at high settings, that is at middle of the road settings. I drop them down, it is still the same. It isn't the graphics or anything else that is the problem, it is because each turn it works out what every AI unit does, every AI city does, it works out what your automated units do, what happens with your resources, if any random events happen, what happens with your buildings that are currently being built... Initially, at the start of the game, it is less than a second per turn, after maybe 20-30 minutes, there is so much going on it is about 15-20 seconds per turn, after an hour or so, you are looking at 5 minutes from the end of 1 turn to the end of another.
With the games he is playing, a quad core is needed. RTS games rely much more on CPU/memory capacity/clocks than they do on graphics, and just about all RTS games out now are multithreaded, so do take advantage of the extra cores