A Copilot/Cursor-style assistant lives or dies on the latency budget. The strong answer spends every millisecond of a 200ms end-to-end budget on purpose, separates the inline-ghost-text path from the chat path, and shows why prefix caching and speculative decoding are the only ways to hit it at 10k concurrent users.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
A Copilot/Cursor-style assistant lives or dies on the latency budget. The strong answer spends every millisecond of a 200ms end-to-end budget on purpose, separates the inline-ghost-text path from the chat path, and shows why prefix caching and speculative decoding are the only ways to hit it at 10k concurrent users.
The screen is whether the candidate budgets latency explicitly instead of hand-waving. The 200ms number forces a split: inline completions are a different system from chat, with a small fast model, aggressive debouncing, and prefix caching, while chat tolerates seconds. Watch for the candidate who routes every keystroke to a frontier model and never mentions cancellation, which would melt the GPU fleet and miss the budget by 10x.
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