PS - I should add, LightMix itself doesn't have a big impact, but, you may now be adding lights into the scene that you wouldn't without it; so before you may have had a night scene with no environment map outside and no sun but lots of interior lights, and a daytime with no interior lights and the hdri and sun, and now you will have all of those in the one scene (and possible extra suns, and extra HDRIs too!). Naturally, doing all those in one scene will be slower to render than the "just the nightime version" or "just the daytime version" because you are doing both at once (but naturally, easier to do them all at once than maintain two scenes and kick off rendering separately, and total render time of the two separate scenes would be pretty comparable to the one render time for the LightMix version).
The only other thing that may have an impact is if you choose to turn off Adaptivity for the LightMix, things may then be a little slower without it, but it will let the render calculations be more evenly shared between each light source rather than giving focus to particularly bright ones (which can be problematic when you turn off the bright ones to leave the dimmer ones). The other alternative there is to make each light source a very similar intensity, safe in the knowledge that you'll be doing the adjustments to intensity in LightMix anyway.
