It’s not so much a hack as a recompilation, but it’s distributed in patch form so I’m accepting it. A person identified as “Nintendo 64 Wizard” took the source code created by decompiling Super Mario 64, and, simply, did something that Nintendo didn’t do: compile the game with -O2 optimization turned on. The result is a much more consistent frame rate.
If optimizing Super Mario 64 is an appealing concept to you, you might be interested in some of the videos made by Youtuber Kaze Emanuar, that goes into why the game has lag, and his own efforts into improving it.
Super Mario 64 Reduced Lag hack (romhacking.net)