After reading toms Elder Scrolls V: Skyrim: PC Performance, Benchmarked at http://www.tomshardware.co.uk/skyrim-performance-benchm... I notice from the chart below and the text that it doesn't scale much beyond 2 cores but the i5 does much better than the i3 at the same clock speed WHY? Is hyperthreading hurting it? The small difference in L3 cache can't make that much difference, any ideas?
To me it looks like Skyrim scales up to 3-4 cores, but Phenom II isn't fast enough to show much difference.
Higher IPC is better for games mostly due to faster processing of threads, including the graphics dependant threads, which can increase FPS in certain situations.
Just because a core isn't at 100%(registers being full) doesn't mean that it is keeping up with producing frames. Each cpu has its frames limit based on IPC, clockspeed, the game's engine, and the complexity of the scene. Getting the other threads out of the way of the graphics dependent threads will help increase framerates, but there is still a limit.
The Core series being much about 40% higher IPC would make the game about 57fps, if the graphics don't bottleneck, compared to Phenom II X4. It scales really well up to that.
I don't think there is enough information to decide why this occurs, nor do I think the author of the article had sufficient information to state that it doesn't scale past 2 or 3 cores.
From that chart you can see that with AMD processors you see 7% scaling going from 2 to 3 cores and no significant difference past 3 cores. I believe this is sufficient to say it doesn't scale past 2-3 cores on current AMD processors.
However, with the Intel processors there is simply not enough information to make a conclusion. It could be that Skyrim will benefit from more than 2 Intel cores thus explaining the difference. It could also be that the game is making poor use of the hyperthreaded cores and that is causing the slowdown. There is also no comparison with a 3 core Intel processor.
To see how it scales on Intel processors it really should be tested on 2, 3, and 4 core processors at 3 ghz. I'm not sure what the best way to accomplish this is. Simply using a 2500k and setting the affinity in task manager may do the trick or you may be able to disable individual cores in the BIOS.