Is your SD card formatted "the right way" ?
I have no idea what's "the right way" but when my O3DS's SD was formatted with GW's menu and my N3DS's SD was formatted with Windows (both Samsung EVO class 10), the N3DS would take several hours to install even a 200 MB file while the O3DS would take minutes for the same thing. And when I formatted the N3DS's SD with GW's menu, the N3DS's install speed became reasonable. So I guess there is a right way and a wrong way.
I have no idea what's "the right way" but when my O3DS's SD was formatted with GW's menu and my N3DS's SD was formatted with Windows (both Samsung EVO class 10), the N3DS would take several hours to install even a 200 MB file while the O3DS would take minutes for the same thing. And when I formatted the N3DS's SD with GW's menu, the N3DS's install speed became reasonable. So I guess there is a right way and a wrong way.