Rate Limits and Usage Tiers
To preserve service levels, and prevent abuse, access to the API is rate limited.
Anyone may use the API without the need of an API key, but will be rate limited to one request per second (over the space of each minute).
If you wish to raise this limit, then sign up for the developers mailing list to get a beta-request API key. This will raise the limit to 10 requests per second.
If you wish to continue to raise the request rate, then there are paid programmes available.
Check out the pricing page for more information