Most sites that need help are built on a theme somebody bought, extended by several people who have since left, on a platform chosen for a business that has changed shape since. A redesign puts new visuals over that. The slow pages stay slow, the parts your team cannot edit stay uneditable, and the reason it was hard to change stays exactly where it was.
Rebuilding is not always the answer. If the foundation is sound and the problem is genuinely presentation, say so and we will tell you that. It is the right answer often enough that it is worth asking the question before spending money on paint.