I found my old gameboy color the other day and popped in 2 AA batteries and played a good hr or 2 of pokemon red. Obviously the graphics don't hold up that well (yet it holds up better than I thought compare to modern pokeman -.-"), but the most noticeable thing is the game is snappy as hell. You open a menu, press an action, walk a step, it just happens. No waiting half a second-1 second for the action to happen. I feel like the portable games strayed away from this after the gba era. 
 
Anyone remember psp era before you could do 'install data'? Remember Midnight club DUB with it's 2-3min loading? Same thing happened to ds.  I couldn't stand pokemon diamond/pearl because of how damn sluggish everything was. The game didn't even look much better than the previous generation (the polygonal map looks arguably worse than the sprite based maps) yet was still about 1/3 slower. 
 
I recently watched someone mess around with 3DS and those menu transitions take wayyy too long.  I kinda doubt NGP will have amazing loading either. Sony consoles are great, but loading times aren't their strong point. 
 
I can see how people are saying ios devices are 'future of portable gaming'. The games are pretty enough (retina display helps). They are also mostly pretty snappy. And you can jump in and out whenever you want with quick app switching (I guess you kinda can with sleep mode on ds/psp).