Bottleneck
A bottleneck is the single component in a system that limits its overall performance under load — a slow database query, an under-provisioned server, a single-threaded piece of code — such that fixing anything else first won't improve throughput until the bottleneck itself is addressed.
Systems typically have exactly one binding bottleneck at a time, even if several components are individually slow — the system's overall throughput is capped by whichever one hits its limit first. Optimizing a component that isn't the current bottleneck produces no measurable improvement, which is a common source of wasted performance-tuning effort.
Finding the real bottleneck usually means profiling under realistic load — CPU, memory, database connections, network I/O, thread pools — rather than guessing, since the actual constraint is often not the component developers intuitively suspect.
