For performance's sake (and to prevent abuse), requests to the API are throttled. Also, identical requests are cached for nominally 60 seconds (most routes, not all).
As a rule, if you're making more than 30 requests over 5 seconds you risk being blocked. Registered applications can make up to 10k requests a day, per IP (300 requests a day for unregistered apps).
Be aware that IP sharing (multiple applications on a single machine, or using a cloud hosting service) can impact an [app]s throttle limits. Thus, we suggest always making the absolute minimum number of requests needed to perform a function.