Blender constantly compositing with smoke in scene

Hello, I hope this should be as simple question for those using smoke effects a lot;

I have a Blender scene with Smoke emitted from particles.

As soon as I open the file, my CPUs max to 100%. As it does this the ‘Compositing’ taskbar is seen at the top but never progresses. If you click the ‘x’ to stop the taskbar, nothing happens, it doesn’t stop and the CPU continues to max out.

I THINK that eventually it does stop, but at a guess it takes an hour.

Does anyone know what this is, if it is necessary and of not how to stop it?

When I first started doing smoke in my scene it didn’t do this, but I have since upped the Smoke Material resolution.