It largly depends on how sharp the edges of the holes or slots are. If they are well champhered it should not make much difference.
Personally, I don't care for them. They provide a solution for a problem that no longer exists. They helped clear a barrier layer of gas that could build up with agressive braking on organic brake friction materials. With today's brake compounds, there is no significant gasing of the friction material that needs to be cleared off.
In my opinion, the only reason for drilled or slotted rotors is for looks, but using them creates other problems.
