Below is my quickie rundown of how well the NVIDIA Shield Portable emulates classic consoles.
Gameboy Advance: Great
N64: Good – the Shield hardware is more than enough to run the games, the emulators available are the problem.
NDS: Good. Some lag in parts, but nothing too bad.
GameCube and above: Poor – games are unplayable
tl;dr: The Shield Portable is best handheld emulator console out there.