No there is no speed cap except what is posed by the ISP you use. When you have multiple downloads of the same file, the servers it's on can only use so much bandwidth. Even though it's generally uploaded to multiple servers, they can only handle so much traffic.
So when there's a large amount of download requests of the same file going on, your speed goes down because everybody shares the same collective speed. So the more people that want the same file simultaneously, their speed goes down until there is less demand on the server for the same file. Then your speed goes up.