I have an AMD FX-6200 (3.8GHz, six cores) with a Radeon HD 6870 (1GB VRAM) and 8GB RAM. Playing Oblivion on maxed details I get around 10 FPS when standing in the middle of Niben Bay and looking across the water. In the Elsweyr/Anequina desert I get even less, so it's not just a weird problem with reflection or something.
I know that Oblivion is pretty heavy on the CPU and not really optimized for multiple cores. However, it doesn't even seem that Oblivion taxes any core to its fullest. I monitored the total system load on the hardware over a few minutes of playing and these were the results:
CPU: 80%, 75%, and 40%; the load on the other 3 cores was negligible. Those are the peak values, so at least 20% headroom there.
RAM: 4GB usage, with another 4GB headroom. I have enabled LAA for Oblivion, and when I play for longer it goes up to 5GB. So this doesn't seem to be the bottleneck either.
GPU usage: 10% peak; clearly this isn't the problem.
GPU memory: 800MB used, with 200MB headroom.
So what's the bottleneck? It can't be the HDD since then I would get stuttering while loading areas, but once they are loaded into the RAM it should run fine, right?
I should probably mention that I have a ton of mods installed (no ENB though) but while I am aware that this slows the game down, considering that this is a comparatively new machine 10 FPS still seems far too little. Especially since I can run maxed Skyrim just fine and I can't find a bottleneck anywhere.
So what's wrong and how can I fix it?