RetroArch is only a front end for other emulators.
The efforts of the libretro team is mainly to get as many cores as possible running. To do this, they add some glue code to enable the emulator to run using RetroArch.
But the emulator itself is still maintained upstream by the original creator. So this is where you need to send your requests for optimisations and improvements.
Very rarely, somebody interested will add some new features to emulators, but it's not the libretro team. But they can still make improvements to the front end that will benefit cores. Such as graphics drivers that make use of the platform CPU, or run-ahead capability to reduce lag. However, I'm pretty sure that the libretro team currently lacks anybody actively interested in maintaining the 3DS port.
FWIW I've added features to the fMSX core and am currently improving the PX68k core. My work benefits all platforms.