Slight would be if the said person doesn't have a muscle memory built up for mice but I think a lot of people are forgetting its 2018 and a lot of people use mice on a daily basis. Game and normal use aren't really that far apart as for muscle memory, ones a cross-hair and ones an arrow. Not that different. The advantage varies for the person. People who use mouses more will have a better advantage, those that don't will get a slight one. I use a controller and hate K&M users but I wouldn't doubt if I used one I would get a pretty big advantage as I have a solid muscle memory for them.