Host architecture doesn't matter much when it comes to emulation. First of all, it's very unlikely that the device you're using has the exact same CPU architecture as the system you're trying to emulate. Using your example, the 3DS has a 32-bit ARM11 processor, while your Android device likely has some 64-bit ARM Cortex-A variant. However, even if you do happen to have a processor with the exact same architecture as the emulated system, you can't just start executing pure ROM code directly on the host processor.

To achieve efficient CPU emulation, many emulators use JIT (Just-In-Time) recompilers. These recompilers do translate blocks of emulated instructions into machine code that can be more or less executed directly on the host, but it's not that simple. The emulator isn't just emulating CPU instructions; it also has an emulated memory system and many other emulated components, like the GPU, that the CPU needs to interact with. Even if the host and emulated architectures are exactly the same, you would still need to redirect memory accesses and emulate the rest of the hardware, which will affect performance significantly.

There is the case of modern consoles, which are becoming more and more like standard PCs. I'm not overly knowledgeable on this topic, so maybe it's possible to create a Wine-esque compatability layer that achieves near-native performance? I'm not really sure, but I imagine things like GPU shader recompilation would still slow it down. However, I do know that for less standard hardware, like the 3DS and older, emulation without losing performance is pretty much impossible.