Working on a streaming chat product taught me something: the standard latency metrics don't really describe what users experience. They're not waiting for a page to load or an API to return a JSON blob. They're watching tokens appear — and what they feel before anything appears is the thing most teams aren't measuring.
That thing is time-to-first-token. TTFT.
There's a particular kind of hell reserved for debugging LLM-backed systems that nobody bothered to instrument. You've got a request that took twelve seconds and you don't know if the slow part was your retrieval pipeline, the prompt construction, the Bedrock call itself, or the post-processing that turned the model's output into something you'd actually show a user. You have logs. You have vibes. You have, essentially, nothing.
We hit this early on an LLM project and it focused the mind quickly.