At what point does system latency typically stop feeling 'instant' and begin to feel like a noticeable delay to a human user?

Research into human-computer interaction (HCI) has established critical thresholds for perception. Interactions that occur in under 100ms feel 'instant' to the user; delays between 100ms and 1 second are noticeable but generally allow the user's flow of thought to remain uninterrupted. Beyond 1 second, the user begins to feel as though they are waiting for the machine. In high-performance systems like search engines or financial trading platforms, maintaining latencies well below the 100ms 'gold standard' is a primary product requirement.