Content Delivery Network.What is CDN ?
They are basically file servers replicating content on a number of locations known as 'points of presence' (data centers), where your ISP will also probably be connected, and where both will share network traffic in the most efficient way possible (direct connections).
Because they can't / don't want to have their own servers in thousands of locations around the world, most companies rely on CDNs to provide content to end users, so that latency is kept minimal and hence improve download speeds on the user's side.
Now, if your ISP has bad connection to these CDNs, your download speeds will suck.