Traditional inventory management:
Move thumb slightly to press button to bring up inventory. Move both thumbs slightly to navigate and make inputs. Maybe a finger or two also to use shoulder buttons if convenient. Move thumb slightly to press button to return to game.
Gamepad:
Move entire head to look away from screen. Move entire arm to release grip on one side of Gamepad, which if not braced against your lap or some other surface necessitates additional effort to grip the pad securely with the other hand. Move entire arm to either remove an additional stylus for input as intended, or use nail to tap at screen, requiring comparatively large movements from your entire arm. When finished, move entire arm back to allow gripping the Gamepad to use in the same manner as you were previously to control the rest of the game. Move entire head to look at the primary screen to return to game.
I mean, I get that you can move slightly faster and potentially make quicker inputs with the second touch screen, and that worked well for Zombie U, even if I felt nothing was lost when the mechanic was translated to the traditional same screen method for it's ports.
But the fact is you simply have to put in vastly more effort to achieve the same results, as well as using an entirely different means of interfacing with the game in doing so.
And that's without even getting into the need to use the terrible low res, poor image quality and less than 100% accurate resistive touch of the Gamepad's screen over you're inevitably better quality TV.
More power to you if you prefer it, but it's still a less efficient way of accessing and managing inventory than just pressing a button and using a controller in the same manner you use it for everything else.
This is simply, factually wrong. Assuming the Gamepad interface isn't somehow horribly broken, it's waaaaaay faster then bringing up a menu or pressing a button to cycle through a list.
I said more effort. Not faster. And even then, given how quick I get with using inventory with games by the end of them I'd argue there's not necessary that big of a speed increase.