When a compiler is compiling a program, and you make a function call (lets say `int x = obj.compute()`), the compiler has to know where the code for that function is. In C (and C++ to an extent), this is easy, functions aren't very fancy and compiler can just go to that place in memory which doesn't change during runtime. These types of functions are called "static functions" and the compilers method of calling these functions is called "single dispatch". Since the code that runs is very predictable, its easy for the compiler to optimize the function call.
In other languages (like Ruby, Java, Swift), a statement like `int x = obj.compute()` has multiple meanings if `obj` can be subclassed. For example, if you have an inheritance structure like (Dog, Cat) > Mammal > Animal, `obj.compute` could mean the compute function on Animal, Mammal, Dog or Cat. At compile time, the compiler may have no idea of knowing which definition of `compute()` to call. So during runtime, it will analyze the type information and call the correct function. These are called "virtual functions" and they are called with a "dynamic dispatch". Because there are multiple functions that could run, actually predicting what will run on the machine is hard.
Dynamic Dispatch function calls are generally slower than static ones, and a compiler would prefer static calls. Chris goes over the many different methods languages attempt to make dynamic calls faster - and Swift, thanks to its simple programming model, (AFAIK) gets "smart" about function calls and can make single dispatch calls when needed (Java can do this to, but what makes Swift "cool" is that it doesn't need a Just In-time Compiler (JIT) to do so).
What all this means to you (the end user) is that you can use all these fancy functions in Swift without having to worry about performance. The overhead of a function call may be something you have never considered (I certainly don't think about it), but I'm sure its something compiler geeks obsess over.
I may be 100% wrong here, I'm not a compiler author, can only make a guess from C/C++ experience, and I've never used Swift before.