API is noticeably slower for me, sometimes up to 10x slower.
Upon some digging, it seems that part of the slowdown is due to the gpt-5 models by default doing some reasoning (reasoning effort "medium"), even for the nano or mini model. Setting the reasoning effort to "minimal" improves the speed a lot.
However, to be able to set the reasoning effort you have to switch to the new Response API, which wasn't a lot of work, but more than just changing a URL.
Upon some digging, it seems that part of the slowdown is due to the gpt-5 models by default doing some reasoning (reasoning effort "medium"), even for the nano or mini model. Setting the reasoning effort to "minimal" improves the speed a lot.
However, to be able to set the reasoning effort you have to switch to the new Response API, which wasn't a lot of work, but more than just changing a URL.