My subjective personal experience is the exact opposite of yours, GPT-5-codex is super slow and the results are mediocre at best. I would probably stop using AI for coding if I was forced to use GPT-5-codex.
I find there's a quite large spread in ability between various models. Claude models seem to work superbly for me, though I'm not sure whether that's just a quirk of what my projects look like.
I don’t think it’s just a quirk. I’ve tested Claude across Java, Python, TypeScript and several other projects. The results are consistent, regardless of language or project structure, though it definitely performs better with smaller codebases. For larger ones, it really helps if you’re familiar with the project architecture and can guide it to the right files or modules, that saves a lot of time.
GPT-5-high (haven’t tried codex yet) is dog slow, but IME if you start with asking it for detailed requirements in a markdown doc with alternatives for each major decision and pseudocode implementations with references to relevant files, it makes a great prompt for faster a model like sonnet.
