10.1Why LLM Load Testing Requires Special Attention

Traditional load testing assumes predictable response times and linear scaling. LLM endpoints violate both assumptions:

• Variable response times. A simple query may complete in 1 second; a complex reasoning task may take 30 seconds.
• Token-based rate limits. You can exhaust your rate limit with a few large requests or many small ones.
• Cold starts. Serverless LLM deployments can add 5-30 seconds to the first request after idle.
• Provider-side queuing. When the provider is under load, requests queue server-side, making client-side concurrency irrelevant.
• Non-linear cost. Unlike traditional APIs, each LLM request costs money proportional to token count. A load test that sends 10,000 requests to a frontier model (GPT-5.5, Claude Opus 4.8) can cost hundreds of dollars.