[UPDATE - Swift 1.2 from Xcode 6.3 Beta 2 brings performance benefits to non-final properties and methods making final unnecessary for performance from that time - see my initial reaction to Beta 2. I still recommend final where possible as it avoids need to consider effects of an object being subclassed and methods overwritten that change the behaviour.]

This post is intended to quantify, explain and show the performance hit that you take from not adding the word final to your classes (or their properties and methods). The post was triggered by a blog post complaining about Swift performance and showing some performance figures where Swift was significantly slower than Objective-C. In the optimised builds that gap could be closed just by adding a single final keyword. I'm grateful to David Owens for showing the code he was having trouble and giving me a base to demonstrate the difference final can make without it being me cherry picking any code of my choosing.

As with most performance issues this doesn't matter most of the time. Most code is waiting on user input, network responses or other slow things and it really doesn't matter that much if the code is being accessed a few times a second. However when you have got performance critical code and in particular that inner loop that is being executed hundreds of thousands of times a second it can make a huge difference.

What does final do

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I've just read Andrew Bancroft's post about being explicit about types in Swift which I largely agree with and is worth reading before the rest of this post which adds an extra reason to follow his advice.

Summary of Andrew's Post

Where the type of a declared variable is not immediately visible from the surrounding code such as when it is declared to equal the result of a function, possibly a function defined in another file, you should explicitly declare the type for human readability and understandability purposes even the the compiler is happy inferring the variable's type. I agree with this.

The Other Big Benefit

That post however misses an entire other benefit of the practice which is that you get the compiler to check for you that the type that will be assigned is he one that you expect. If the function you are calling gets changed you will get an error at the call site not where you first use the object of inferred type (if you are lucky - you may get no error at all if the object is used in locations where type is liberal such as string interpolation).

Consider Defining Your Variable as a Superclass or Protocol 

Now it may be possible to declare your variable to a superclass or protocol of the expected object's type and if so that should be done to give some flexibility while ensuring expectations are met. This also documents the flexibility of the type that you can accept. 

Drawback of Explicit Typing

The disadvantage of explicitly typing your object is that if you change what the function that sets it returns and it is still compatible you have to manually change the type (possibly in many places if it is a heavily used function). I think that is a price worth paying to encourage at least a cursory check of the use of the values and to ensure it is not just well typed (which the compiler can do) but also correct.