when should you think about using a CDN for your static assets? like, its not always necessary, right?

hey folks, I'm working on a web app that's got more and more static stuff like images, CSS, and JS. it makes me wonder when we should really start thinking about a Content Delivery Network. we're not huge yet, but the site's loading a bit slower these days. what do you all think is the right time to get a CDN. is it when it's absolutely needed, or just for a little speed boost