The fact that its performant is tangential to the fact that its running on ARM android. IIRC Drastic still does a traditional JIT to recompile the guest ARM code into ARMv7a for phones (I might be misremembering, but I recall they had a whole lotta headache trying to update to support 64 bit ARM in time for the Android deadline for that. Also I remember them talking about tons of headaches dealing with the new storage APIs for Android)

Its really fast because of the tons of optimization that went into the application. It does many cool things to cut out overhead on the CPU side, but also it does a lot on the graphics emulation side, including hand rolled ARM NEON code for SIMD processing polygons. I'm not very familiar with DS emulation, but one of the devs for Drastic contributed to another project I worked on, so I had some chats here and there about these sort of things.