Short answer
There’s no comparable benchmark here that establishes one GPT model as the fastest for every production workload. OpenAI describes Ultrafast as its fastest API service tier. It supports GPT-6 Astra and GPT-6.1 Sol through the Responses API, with WebSocket support and an HTTP alternative. OpenAI says to use Ultrafast when the speed justifies its higher cost. That is a tier-level claim—not proof that either model is fastest for every request. (Ultrafast mode | OpenAI API)
Practical starting point: For Ultrafast, compare Astra when a task calls for the family’s hardest reasoning and Sol for complex coding, research, or computer use. For focused, high-volume work, evaluate GPT-6 Luna through the Responses API. These are workload recommendations, not measured speed rankings; OpenAI’s Decisions API announcement compares its bounded-decision service with Luna through the Responses API. (A model guide for the GPT-6 family)
Which fast GPT API option fits your workload?
| Your priority | Candidate to evaluate | What the available guidance says—and what to check | |
|---|---|---|---|
| Fastest advertised service tier | Ultrafast with GPT-6 Astra or GPT-6.1 Sol, through the Responses API | OpenAI calls Ultrafast its fastest API service tier and names Astra and Sol as supported models. The guide describes WebSocket support and an HTTP alternative. It says to use Ultrafast when speed justifies the higher cost. Astra is positioned for the hardest reasoning work; Sol for complex coding, research, and computer use. These positions don’t establish which model is faster on your workload. ([Ultrafast mode | OpenAI API](https://developers.openai.com/api/docs/guides/ultrafast-mode)) |
| Focused, high-volume work | GPT-6 Luna through the Responses API | OpenAI positions Luna for fast, efficient everyday work at scale. Confirm the model and configuration are available for your account. This positioning is not a workload-specific benchmark. (A model guide for the GPT-6 family) | |
| Classification, routing, or other bounded decisions | Decisions API | OpenAI says the Luna-built Decisions API is designed for bounded decisions and can be up to 10× faster than GPT-6 Luna through the Responses API. That claim is about the described decision tasks, not general chat or arbitrary generation. (Build more responsive apps with the Decisions API) |
Treat these as candidates, not a guarantee that one option will be fastest for your prompts, integration, traffic, or quality target. Check model and tier availability for your API account before planning a rollout. (Ultrafast mode | OpenAI API)
How to tell which is fastest for your production API
Treat advertised speed as a shortlist, not a benchmark result. Measure the candidates available to you using requests that resemble your production workload:
- Choose representative inputs. Include typical and unusually long prompts, the tools or structured output your application uses, and realistic response lengths.
- Hold conditions steady. Use the same inputs, output limits, streaming behavior, region, connection setup, and concurrency for each candidate wherever possible. Record differences you cannot hold constant.
- Repeat the runs. Compare distributions, not a single request. Record time to first token and end-to-end response time separately; also track output rate if streaming speed matters to users.
- Check quality alongside speed. Score whether each response completes the task correctly. A faster result that fails more often may not improve the application.
- Report the setup and result. Name the model, service tier, API, test conditions, sample size, latency measures, and task-quality result. If you have not run the test, describe the recommendation as qualitative rather than declaring a measured winner.
This is a proposed evaluation method, not a test performed for this article. OpenAI’s GPT-6 guide recommends measuring task success and latency when preparing for production. (A model guide for the GPT-6 family)
Bottom line
For OpenAI’s fastest advertised API tier, evaluate Ultrafast with GPT-6 Astra or GPT-6.1 Sol through the Responses API—and use it only when the speed justifies the higher cost. Consider Astra for the hardest reasoning work and Sol for complex coding, research, and computer use. For focused, high-volume work, evaluate GPT-6 Luna through the Responses API; for bounded decisions, consider the Decisions API. These product descriptions do not establish a measured winner for your workload—compare eligible candidates under matched conditions. (Ultrafast mode | OpenAI API)