Well looks like there are some performance issues:
LibreELEC + Retroarch 9.80 for Raspberry PI 4 !
As far as I can tell it's somewhat based/cherry picked on older packages of my fork so I can't tell how recent the packages are. In theory the RPi4 should be handled like any other mesa based build but all libretro cores probably have to be adjusted for armv8-a. Of course these flags were set by the project before but I'm not sure if they were overwritten by RPi platform flags.
Btw. can you test my PR if it works for you? Makefile: added devices for Amlogic & Rockchip platforms by 5schatten · Pull Request #74 · libretro/mupen64plus-libretro-nx · GitHub
I guess once it works and is ready for common usage he'll post some code to compile it on Linux too.
One question, the G12 devices, also includes s905x2? Since in libreelec's build it is like that. Sorry for bothering