UHS-1 speeds require a special bus - the 3DS does not have this, so you'll only ever get non-U1 speeds. Not sure whether a Class 10 SD card would be faster or slower than a cartridge though.

 

Well, only one way to find out.

Can someone who has super smash bros on a cartridge take a timer from the second you press the game icon to the game menu appears?

then i will do the same with my 40MBs sdcard and see how fast the cartridge really is. (and calculate it's read speed on the old 3ds (calling it old since the new arrives tomorrow)

 

All I know is MH3U was held back by the console itself and not the storage. On the N3DS it loads over twice as fast as it did in the 3DS, and that's from a slower SD card (C10 16GB SD to C4 4GB mSD)