ChatGPT 5.4 Pro Standard Mode - Adaptive Thinking or Nerfing Model?

Just wanted to add that I have also been experiencing a significant regression of the pro models. Thinking time is the most obvious signal, but as the previous users have said, I have also noticed significant decline in model performance. For me, my usage is mostly in mathematics, and I have noticed much more common misunderstandings and hallucinations in my usage. This is concerning because the pro subscription is costly and I am currently not getting the performance that justifies the cost. Hopefully this gets resolved soon.