Looks like IPtoASN is useful to you. Great!
However, the public API is not really designed for heavy users.
Still want to use it for your service? That is totally fine, but you should rather download the databases, and run the API server on your own infrastructure instead.
All you need to achieve this is download and install iptoasn-webservice, and enjoy a fast API without any restrictions.
If you still prefer me to host the service for you, I would kindly ask you to contribute to the hosting fees.
Cheers!
-Frank.