All hotkeys, including the ones related to execution speed, can be changed in DeSmuME > Preferences, and clicking on Input at the top. The relevant options for changing execution speed are "Set Speed" and "Enable/Disable Speed Limiter".
If nothing else works, you can reset Mac DeSmuME's settings to their defaults. Reinstalling the app itself does not do this -- only deleting the org.desmume.DeSmuME.plist file can do this. (This is the standard behavior for Mac apps, and thus this behavior will not change in the future.)
The org.desmume.DeSmuME.plist file is located at: ~/Library/Preferences/org.desmume.DeSmuME.plist
If you find this too difficult, then you can run "Delete DeSmuME Preferences.app" that comes with your Mac DeSmuME installation package, which will delete the org.desmume.DeSmuME.plist file for you.
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