Which of the following is NOT a common sign of a network security breach?

Faster internet speed is typically not associated with a network security breach. In most scenarios, a security incident would lead to degraded network performance due to increased malicious activities, such as data exfiltration or denial-of-service attacks, which can consume bandwidth and resources. Other common signs of a breach, such as unexpected changes to files, increased network traffic, and unfamiliar devices on the network, often indicate that unauthorized access or manipulation is occurring, while faster speeds could imply optimal functioning rather than a breach. Thus, faster internet speed stands out as an indicator that does not fit the context of signs typically observed in a network security breach.