when's the right time to introduce a cdn

Hey everyone, I've been noticing a lot of talk about performance lately, and it got me thinking about Content Delivery Networks. We've always just served our assets from our main servers, but as traffic grows, especially with users all over the world, I'm wondering when it makes sense to actually implement a CDN. What are the main things you look at? Is it just about latency or how much traffic we're getting, or are there other things like saving money or taking some load off our servers that make you decide to go with one?