I am getting the same behavior. Simple tasks in agents api with 3-4k tokens are taking 70 seconds
edit: I think I may have found part of the problem. even with verbosity and reasoning at their lowest settings, gpt 5 has a tendency to write a LOT more tokens than 4.1.
If you specify the length of the output, it equalizes somewhat. I’m still finding large structured outputs are significantly slower on gpt 5 though.