Latency
How long a request takes — the difference between an assistant that feels alive and one that feels broken.
For LLMs, latency splits into time-to-first-token, which drives perceived speed, and total generation time. Streaming attacks the first; smaller models and shorter outputs attack the second. Reasoning models trade latency for accuracy on purpose.
In practice: 400ms to first word feels instant; four seconds of blank screen feels dead.