A modder / game designer by the name of Kaze Emanuar has been working with the original source code of Mario 64 and improving it in order to have the game run better on original hardware. I found this series, which is still not concluded, absolutely fascinating, as it highlights the importance of well optimized code and knowledge of the machine you're coding for.
Kaze has for many times stated that the original team was unaware of certain QOL improvements that could be deployed, as they were inexperienced with the console and engine while developing the game.
Please find the videos below, in release order. I'll try to keep these updated as much as I can.