Recalbox 2021 defaults to for DS games. However, DeSmuME is CPU-intensive. For Metal Slug 7 , which has heavy 2D sprite rendering:
Since Metal Slug 7 is a Nintendo DS exclusive, you should use the following cores within Recalbox: metal slug 7 rom mame recalbox pc 2021
, which is the enhanced "Director's Cut" of Metal Slug 7. It was released for the PSP and is easily emulated on Recalbox PC using the Recalbox 2021 defaults to for DS games
was never in arcades → no MAME driver. metal slug 7 rom mame recalbox pc 2021
If you specifically want the original 2008 Nintendo DS experience—which used the bottom screen as a live map—you will need to bypass MAME entirely.