Recursion has a large amount of overhead as compared to Iteration. It is usually much slower because all function calls must be stored in a stack to allow the return back to the caller functions. Iteration does not involve any such overhead.

Why is recursion so slow?

Recursion is slower and it consumes more memory since it can fill up the stack. But there is a work-around called tail-call optimization which requires a little more complex code (since you need another parameter to the function to pass around) but is more efficient since it doesn't fill the stack.

Is recursion always faster than iteration?

Memoization makes recursion palatable, but it seems iteration is always faster. Although recursive methods run slower, they sometimes use less lines of code than iteration and for many are easier to understand. Recursive methods are useful for certain specific tasks, as well, such as traversing tree structures.