Volumetrics always add some time, and usually quite significant, since the renderer is doing a lot more calculations. Are you using Inside Mode?
For speed, the first thing to check is "Single Bounce Only". If you can enable this and still get the result you want, then it will dramatically reduce the amount of extra calculations the engine has to do.
The second biggest control for rendering speed there is the Step Size, which determines how large a step through the volume is taken before calculating again. Smaller steps mean more accuracy, but of course also mean you are asking the render engine to do many more calculations. There's no good guide to what Step size to use, since it will depend on too many scene specific factors, like the scale of the scene and the amount of detail in the volume (when using 3D noise).
Since it looks like there is no 3D noise here and the fog is uniform, you could try a larger step size. You might also be able to use "On Surface" mode (where "Single Bounce Only" still applies, but Step size doesn't as the engine does not step through the volume recalculating things).
