firm file in the /luma/payloads/ folder and hold START to load up the chainloader to launch it. One major issue is there's a chance that it may not detect the correct save format or will corrupt the existing save file so make sure to backup your saves beforehand (they are currently trying fix it by having a better database for save detection, if you're interested you can help).
The major advantage of this compared to VC injection is it can load the ROM through the SD card instead of having to convert and install them, but needs to be launched on boot instead of being able to use the 3DS HOME Menu (there's ways to make a forwarder for in on the HOME menu, but requires additional setup to get it to work).
If you're using an old 3DS/2DS, this is the best option to play GBA games.
Because it's technically running a 'virtual' GBA, it can't provide features that are normally available on other emulators, such as savestates, screen filtering, or sleep mode, however it does have high compatibility with the exception of cartridges with special hardware. The 3DS does include native firmware for a GBA mode which was mainly used to run ambassador releases instead of having to develop an emulator for them. What makes this emulator stand out is the fact that it's able to display the graphics in 3D the way it was intended, something that is almost impossible to do on other platforms, unfortunately compatibility is still low and requires a New 3DS to run.
The nightly builds have seen major improvements since the initial release and can even play some of the more demanding games at full speed (See here for some tips), so it's better to use that until the next stable release.
Generally, 2D games will perform better than 3D, however you can get some better performance by tweaking the settings if necessary.