Gentoo wants to compile from source, because of the USE flags. Thats hard on battery life, and maxes out CPUs creating thermal issues, both of which are notorious issues on mobile.
You should be able to accomplish the same thing by using your own pacman repos with compiled binaries that have the right USE flags. They all have the same hardware after all.
Anyway, from my own experience, the video decoding performance of gentoo-on-rpi-64bit was somehow nowhere close to LibreELEC, on the same hardware. Perhaps there is a lesson to be learnt there...
You should be able to accomplish the same thing by using your own pacman repos with compiled binaries that have the right USE flags. They all have the same hardware after all.