Slow server responses could caused by server-overload, if your server is unable to handle the number of requests Terra is sending, if your server is unable to handle multiple requests concurrently or does not have sufficient resources to manager large amounts of traffic, you should consider scaling your ingestion service (allocating more resources).
Or, there may be long synchronous queries being run on your webhook endpoint (to your database or other) that is stalling the response. Look through any queries you are making and optimise your queries or execute them asynchronously.