Mesa+Nouveau is bad only thanks to nvidia and their signature lock implemented since the 900 series, as even stated by me before:
the open source nvidia driver it’s not able to re-clock the gpu with an higher clock than the boot one (and trust me it’s a really low clock) and you are not able to use a quarter of the power the gpu has.
Even if the open source driver code is 100% equals the nvidia one, literally copy pasted, it would not work because it need to be signed by nvidia to do so.