One trick I used to use with my old SD (which happened to lag with Black/White) was this:
1. Open the DSTWO's menu (L+R+Start by default) while playing Black/White.
2. Set the slow motion to 100%
3. Close the menu and go back to Black/White, and make the screen change (such as opening your pokedex or something). Your game should be super slow.
4. Now go back and set the slow motion to 0%
5. Your game should no longer lag now.
This trick used to fix all the games that were laggy back on my old SD.