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.

I worked in an app where we had ~ 400 slices defined on a cube and we did not see performance issues. But every app is different, and I do believe the more slice you have and the more evaluations you ask OneStream to do, the potential is there for it to impact performance.

Our slices were constructed such that we started by removing all access first here in the first slice on the cube:

behaviorInGroupInFilter="IncreaseAccessAndContinue" accessLevelInGroupInFilter="ReadOnly" behaviorInGroupNotInFilter="SkipItemAndContinue" accessLevelInGroupNotInFilter="NoAccess" behaviorNotInGroupInFilter="ApplyAccessAndContinue" accessLevelNotInGroupInFilter="NoAccess"

Then we added it back via a person's security group here:

behaviorInGroupInFilter="IncreaseAccessAndContinue" accessLevelInGroupInFilter="AllAccess" behaviorInGroupNotInFilter="SkipItemAndContinue" accessLevelInGroupNotInFilter="NoAccess" behaviorNotInGroupInFilter="SkipItemAndContinue" accessLevelNotInGroupInFilter="NoAccess"

And we did not have any "Stop" slices.  All of ours were "AndContinue" and even with 400 slices, performance was okay on that cube.

But again, every app and data model is different so the potential is there.