Oh the irony: your i7 is hardly used but it is precisely your i7 that is the bottleneck. You see, RTW is hardly multi-threaded; and during the battle both the engine and the logic (AI) must share a thread. Since the load on the system increases as the variables in the scenario (i.e. objects, and complexity of the physics of those objects) do you will quickly max out one core and simply run an underpowered single processor system as far as RTW is concerned from then on. Hence only 25% CPU load, but a lagging game nonetheless.

You may be able to run RTW/EB a bit smoother if you look into setting both CPU affinity of the cleanup and the RTW main processes/threads via the task manager (Processes tab).