const stands for constant, and it means the variable cannot be reassigned at a later time.

let is similar to var except that it is block scoped, which means it can be declared inside of a for loop and will be local to the body of that for loop (and therefor does not exist outside of it)

The latter is different from a var variable which can be declared anywhere but is always local to the function scope.

In general it is good practice to try and define your variables as const as much as possible.

I agree with Giorgi that performance is not the main reason. A code analyzer could just as well determine that a variable declared with let is not ever reassigned and optimize it the same as if you had declared it with const. (Heck, linters have rules to detect this and suggest using const instead of let.)

Yes, it does signal to the reader that you're not going to assign to the variable. The benefit of const, over putting a comment saying the same thing, is mainly that const is a standard way of signalling it. Being standard, it transmits the information more readily than custom comments. (Also, a comment could be wrong but const won't let you be wrong.) I don't think this is the main benefit though.

The "principle of least privilege" is often invoked in conjunction with const, but why should I care about the least privilege? Because it helps with early detection of coding mistakes. Consider the following code:

function findSomethingInDocument(document) {
    let tree = getTreeFromDocument(document); // We get some sort of tree structure.
    let found;
    for (const child of tree.children) {
        if (child.someFlag) {
            tree = child; // Oops I meant found = child :(
            break;
        }
    }
    return found;
}

In this code, I typed tree = child when I meant to type found = child. Yes, the bug can be found in testing. But why wait for testing? I never meant tree to be changed. If I had marked it const then I would have immediately learned the error because the compiler would informed me of it. I would not have to wait for testing. The code above is fairly simple but imagine a more complicated algorithm that uses more variables.

Use let when the variable's value can be changed.

Use const when the variable's value cannot/should not be changed.

Do not use var.