Codex Fast Mode vs Claude Fast Mode: What’s Actually Different?

TL;DR Both Codex and Claude support a fast mode, but the way they achieve speed is completely different. Codex has two tracks: either it serves the same GPT-5.4 model about 1.5× faster, or it runs a separate small model called Spark on Cerebras wafer-scale hardware at more than 1,000 tokens per second. Claude keeps the same Opus 4.6 model and speeds it up through infrastructure-level prioritization, with output speed improving by up to 2.5×. The tradeoffs around price, speed, and intelligence retention are subtle, and which option is better depends on your workflow. What got me curious Since I use both Codex and Claude Code, I already knew both sides offered a fast mode. But the pricing felt different, the speed felt different, and the user experience felt different. Sean Goedecke’s post, "Two different tricks for fast LLM inference," made it clear that the two companies were solving the problem in fundamentally different ways, so I started digging deeper. Codex fast mode: really two d