To be exact, the answer is that any revision and subsequent re-release is a 'new gen'. What that change is or what benefits are seen can only be determined by bench-marking/reviewing the hardware in question.
Each piece of hardware will be different. There is no general standard for what performance differences you see, especially if you don't even post what system it is built in to.
Your Q is way too vague to get any meaningful answers.
Even if I assume it is an Intel processor the benefits depend on whether you are using a GPU or the Integrated graphics, and the model of the processor (have different speeds and different HD graphics modules, even in the same series IE 4th gen i7s can be HD4600, HD5000, HD5200).. etc