Throughput
The rate of work a system completes per unit of time — and a quantity that trades against latency rather than tracking it.
The trade is the part worth understanding. Batching, queueing and high concurrency raise throughput and raise latency at the same time; the system does more total work per second while each individual request waits longer. A design tuned for maximum throughput will have poor tail latency, and vice versa.
Little's Law connects them: at a given concurrency, throughput and latency are inversely related. Push utilisation towards 100% and queueing delay grows without bound, which is why systems are run with headroom and why "we have spare capacity, the CPU is only at 85%" is often wrong.
Measure it under a realistic mix, and always alongside latency at percentiles. A throughput number on its own is meaningless — any system can achieve high throughput if you allow unbounded latency.