Today we started getting the throttling error message back (Server encountered exception: Maximum number of threads throttled) due to our increased volume. We’re using the legacy API Key. We currently are having to send 3 calls per new contact: Add, tag & opt-in. Is there a way to combine these to just one API call?
No, the api will count every one as an individual call.
Have you considered using OAuth? It is required for REST but the XML-RPC can still use the api key method or OAuth. Using OAuth will give you a greater number of calls to work with…and if it ends up that you need more, then it can be expanded by the developer support team.
Otherwise, you have a much more limited field to work with using the api key method.