Well you are combining two idea into one, so i will explain each.
ok think of it this way. When you use your brakes, they produce X amount of heat. That heat is spread out over the entire rotor. For example say 30* per cubic inch. Now cross drill and slot the rotors. Now you just reduced the amout of area the heat can spread. You are still producing the same amount of heat, but have less space to spread it out. So now you would have 40* per cubic inch.
The second part is that the slots and holes cause more friction. Therefore more friction = more heat = hotter rotor temps.
I dont remeber where i picked up the info but i’ve read it a few different places.