What is the main problem of the chosen service interaction structure - No caching to speed up data processing - No retry mechanism in case of failure - No adaptation to changes in preferences - Synchronous calls increase delays due to response time dependency - Authorization check issues in API - Error in processing responses from API

The main problem of the chosen service interaction structure is synchronous calls, which increase delays due to dependence on the response time of other services. This approach reduces the system's fault tolerance and scalability, as each service waits for a response from another, which can lead to a chain of delays and blockages.

Other listed issues (lack of caching, retry mechanisms, adaptation to changes, authorization problems, and response handling) are also important, but the key bottleneck is precisely the synchrony of interactions, which directly affects the performance and reliability of the entire system.