What are the benefits of `let` in Swift?

You asked "...are there some performance optimisations the compiler does apply when we use constants instead of variables?"

The answer is yes, absolutely.

Mutable collections may be organized differently than immutable ones in order to allow for them to be changed. Immutable collections can be optimized for read-only operation.

Then there is the use of mutable/immutable objects. The compiler may have to generate code that copies a mutable object when it's shared as a property of another object in order to avoid undesired side-effects.

Comparison of immutable objects (equatable/comparable) can also be optimized in ways that mutable objects can't.

Sulthan's point about the intelligence of the compiler is a good one though. The compiler can often deduce that a variable is never going to change from code analysis, which can make benchmarking let vs. var usage hard.

The correct answer for now is "probably not".

Providing additional information to the compiler is always wise, however, the compiler is already rather smart. In many cases it can see that a variable is actually a constant even if you use var, so saying let won't be any new information and it won't provide a benefit.

The biggest benefit of const/let is a protection against programming errors. It can have some performance benefits in very specific cases but modern compilers don't really need the programmer to tell them that a variable is assigned only once.