For those who cannot be able to save on DeSmuME:- Check the "Path Settings" in Config and see if the save files are in a folder path that exists (in other words... a folder that really exists).
- If you have changed some folders around recently, be sure to change the location in Path Settings.
For those who have DeSmuME running slow on their computer:- Check to make sure that the 3D setting is set to Softrasterizer and NOT OpenGL, because using it on OpenGL is basically running on the computer's graphics card and processor, which would already be used by other applications too.
- Make sure that DeSmuME is not running any Firmware or the Bios for the Firmware, 'cause it WILL slow Pokémon Black or Pokémon White down.
- Make sure that you only have ONE DeSmuME emulator running, having more than one running can also slow it down. Press Ctrl Alt Delete to bring up the Task Manager and click the Process tab and check if desmume.exe is running if it doesn't appear at all, then launch DeSmuME again.
I hope this comes in handy for those who have DeSmuME running slow for Pokémon Black or Pokémon White ROMs.
And I sure hope it also comes in handy for those who can't save on Pokémon Black or Pokémon White ROMs on DeSmuME. 'Cause I did have that problem yesterday, I thought I might as well let you all know for sure.