In the interview with DF if I recall correctly, they mentioned particles physics are cpu driven, and they are a very expensive system for them.
For LightHeaven's reference:
"A lot of the work was making the engine multi-threaded. The original Source engine had a main thread that built up a command buffer for the render thread and there was a frame of delay. We kind of changed that so the render thread buffered commands as soon as they were issued - in the best case, it gets rid of a frame of latency there. And then almost every system has been made multi-threaded at this point. The networking is multi-threaded, the particle simulation is multi-threaded. The animation is multi-threaded - I could go on forever. The culling, the gathering of objects we're going to render is multi-threaded. It's across as many cores as you have."
The arrival of AMD's Mantle API has put a lot of focus on DirectX 11's API and driver overhead, and Respawn's solution to this is simple enough while highlighting that this is an issue developers need to work with.
"Currently we're running it so that we leave one discrete core free so that DX and the driver can let the CPU do its stuff. Originally we were using all the cores that were available - up to eight cores on a typical PC. We were fighting with the driver wanting to use the CPU whenever it wanted so we had to leave a core for the driver to do its stuff," Baker adds.
Going forward, the quest to parallelise the game's systems continues - particle rendering is set to get the multi-threaded treatment, while the physics code will be looked at again in order to get better synchronisation across multiple cores. We'll be seeing an interesting battle ahead in the PC space - the pure per-core throughput of Intel up against the more many-core approach championed by AMD. It's Baker's contention that games developed with Xbox One, PS4 and PC all in mind will see console optimisations result in better PC performance.