Router-First Architecture Beats LLM Cascades for Latency-Sensitive Apps

For interactive applications, choosing between LLM routing and cascading is a critical engineering decision that directly affects latency, cost, and user experience. A predictive router selects a single model before generation, adding only 2–35ms overhead while delivering predictable tail latency and no redundant token processing. Cascades, by contrast, send every request through a cheap model first and escalate to stronger models when a verifier fails — meaning even a 1–3% escalation rate can eliminate most cost savings. Experts recommend a router-first approach for apps with strict response-time requirements, especially when traffic includes clearly classifiable tasks like extraction, summarization, or FAQ responses. Cascades remain preferable only when outputs are easily verifiable and accuracy matters more than tail latency.