when to introduce a cdn for static assets

hey folks, i've been thinking about content delivery networks lately. our app is growing, and we're serving a good chunk of static stuff (images, css, js) straight from our own servers. load times are starting to get a little slow, especially for people far away from our main data center. i'm curious, when do you usually decide it's time for a CDN? do you look at traffic volume, how many assets we have, how slow things are getting, or a mix of it all? would love to hear what you all do and how you decide.