Is there a rate limit or speed limit for API usage?

Anonymous users have a limit of 20 calls per 10 minutes and 50 calls per day.

Authenticated users have a limit of 300 calls per 10 seconds, 2,000 calls per 10 minutes and a limit of 50,000 calls per day. Authenticated users of free data feeds have a concurrency limit of one; that is, they can make one call at a time and have an additional call in the queue.

Premium data subscribers have a limit of 5,000 calls per 10 minutes and a limit of 720,000 calls per day.

Premium data subscribers are limited to 10 bulk downloads per hour.