I agree, but keep in mind all those emulators were pretty much tuned for the NDS, this is not the case with Retroarch cores.
Retroarch itself might be ported and optimized for the 3DS but its emulators cores are likely not going to be. Why? Because this is how it works: it uses highly portable emulation cores that are supposed to be platform-agnostic and not rewritten for each new platform, only retroarch needs to be ported as it handles all the platform-specific stuff.
This is the strength of Retroarch but it's also its weakness when it comes to relatively weak platforms (not only the 3DS, you can look at the existing PSP port to see what it can currently do compared to existing optimized emulators).
That's the reason why you don't have yet another straight snes9x port being developped but something like Blargsnes instead, which is fully optimized for 3ds. That's also the reason why it's pointless to try to run genplusgx core on 3ds because, even if you got it to start a game, it's never going to run anything at decent speed unless you rewrite quite everything in ARM assembly.
Also, I don't think Jenesis DS sourcecode was ever released and don't believe Lordus is going to make a comeback in the 3DS scene after having worked with Nintendo so it's not going to be ported on 3DS.
Our best chance for Genesis right now is to use Picodrive as it has code optimized for ARM CPU. If someone can get it running in Retroarch for 3ds, this might give a good indication on what kind of speed we can expect.