It's the problem with particle effects and physics(They need a very good cpu, usually heavily overclocked help a bit). Tweak them.
As for physics, disable or change the option for physx if there's any.
Shadow emulation can also be tweaked.
Don't stream/broadcast stuff... They are using precious cpu resources to encode/decode the video stream.
Save a running chart of the GPU and CPU load and see which one is bottlenecking (usually spikes at around 80%+) and tweak the relevant settings up or down.