But GPT 3.5 has Turbo but nobody claims it reduced accuracy even though it's much faster.

GPT 4 is now a lot faster and struggles much more. And in theory being faster should be an advantage to an extent, even if the model needs more prodding, as you gain a more seamless back and forth instead of waiting half a minute.

Some people say that people think it's nerfed because when it was slow we associated it with "thinking harder" but I doubt that's the case, and the way to measure that is by loss of ability, i.e. if it is stuck more often as a result of itself and not a bad prompt.