GPT-5 performance degradation on simple tasks

Codex no longer completes tasks reliably — in roughly two-thirds of all cases, tasks either hang indefinitely or end with 'I could not do this task.' It makes a huge number of mistakes and regressions... two-thirds to three-quarters of all consumed limits go not into building, but into cleaning up Codex's own mistakes.

for simpler, everyday queries, some users have reported that GPT-5 can feel slower than the highly optimized GPT-4o. There was even some initial community backlash about performance degradation for simple coding tasks before OpenAI fine-tuned the model routing