Yeah, certainly at the small scale there can be a myriad of reasons why a system doesn't perform. At a large scale its hard to imagine real world scenarios where storage has not been the bottleneck. Regarding whether twitter should be using a database, unless they dont want to store there data they need a "database". Does that mean sql? No. It just means storage. As paul Graham has demonstrated a file system can be a great database. But it still a database.