They will generally be noisier and take longer to clean up than scenes that don't use volumetrics. Also, high directionality lights can have more noise than one with lower directionality, but high directionality may be needed for the light in volumetrics depending on the look you are going for so this could be unavoidable and just need more passes. You could also look into whether Single bounce only was checked and whether you can use that, as that will speed things up greatly (may take as many passes to get rid of noise, not sure on that, but the passes will be faster at least :) )
A higher GI vs AA balance will likely help, since it makes GI and light noise clean up faster (at the expense of anti-aliasing). Denoising will help. You could also look into using Sharpening and Blurring too.
