You can play DS games on the Nintendo 3DS. No problem there. But did you know they take a lot longer to load on Nintendo's new handheld than they did on the old one?
A series of tests run by GameSpot, booting up five games on both the DS and 3DS, found that the new 3D handheld takes much longer to load a game than its predecessor, in some cases (like Black Ops) taking 21 seconds to go from boot to main menu.
While this won't destroy your ability to enjoy older games on a newer handheld, it's interesting that a more powerful machine takes longer to do something than its feebler sibling could manage. Then again, considering the DS games are running essentially as emulated titles - which always taxes hardware more than a game running on its intended device - the wait can be excused!
3DS takes longer to load DS games [GameSpot]