Hi! “Casting” and “Interfaces” are very different things, but I don’t think using any of both features affects performance in any way because they are very basic operations, like getting a reference from an instance. You can look similar thread here: https://forums.unrealengine.com/development-discussion/blueprint-visual-scripting/41578-interfaces-vs-casting-have-a-bit-of-confusion .
And if you need more theory behind Interfaces as a programming language feature take a look at C# interfaces, or Delphi Interfaces to have better understanding how and when to use them.