Spark is the 'same model and harness' but on Cerebras.
Your intuition may be deceiving you, maybe assuming it's a speed/quality trade-off, it's not.
It's just faster hardware.
No IQ tradeoff.
If you toy around with Cerebras directly, you get a feel for it.
Edit: see note below, I'm wrong. Not same model.
> Today, we’re releasing a research preview of GPT‑5.3‑Codex‑Spark, a smaller version of GPT‑5.3‑Codex, and our first model designed for real-time coding.
from https://openai.com/index/introducing-gpt-5-3-codex-spark/, emphasis mine
You're right. It's funny because I kind of noticed that, but with all of these subtle model issues, I'm so used to being distraught by the smallest thing I've had to learn to 'trust the data' aka the charts, model standings, performance, etc. and in this case, I was under the assumption 'it was the same model' clearly it's not.
Which is a bummer because it would be nice to try a true side-by-side analysis.