Does applying slice security have any performance impact? Or is it more just maintenance impact?

Yes, it can have performance impacts (but will likely only see it in larger applications where slice is present) as it has to perform checks on all members to see where the application (slice) security needs to be applied. If there are a lot of members with slice and a lot of slice security groups, I would keep it in the back of your mind. This can impact rendering times on cube views, dashboards, etc. We have played with it in some applications where we were looking at performance concerns and slice was a factor. I want to say there was an app or two out there where it was reported to be a problem, but overall, you don't notice the impact.