Right now the emulator does not have a hardware renderer. Install the emulator and extract the zip file of ROM to run it inside the emulator. This emulator stands out because of its simplicity, thanks to its direct interface and how smooth it makes playing true legends from the world of video games.
With this new release comes brand new Cocoa frontend designed to make playing more pleasant for Mac OSX users. Try applying the latest system update and see if the issue improves. When comparing melonDS and dolphin you can also consider the following projects: desmume - DeSmuME is a Nintendo DS emulator. StapleButter is currently working on creating a hardware-accelerated renderer for melonDS which will greatly improve performance on lower-powered systems like the Nintendo Switch and Android devices.