For hobby development, I'm trying to speed up the unofficial PlayStation emulator on the Nintendo 3DS. There are all kinds of interesting problems there, like SD reads being so slow that it tanks the framerate any time the emulator hits the disk (so I might be writing a read thread + precache?), and some apparent room for hardware-specific optimization in the lighting and blending routines.

It's been fun to work around the constraints on an underpowered device. It's also an excuse to learn ARM assembly, and a nice break from all the JavaScript I've been spending my time in lately!