I've been trying to get Castlevania Adventure Rebirth working this morning but I can't seem to figure it out. I boot up the game as usual but after the main opening cutscene, the game drops to 0 FPS and freezes. Help would be greatly appreciated
Are you on Dolphin 5.0 stable, or the latest dev build?
Your profile says that you are using 5.0 version, which is very old and outdated.
This problem of crashing in Castlevania Adventure Rebirth has been fixed in the latest versions, so download and use the latest beta/development version of Dolphin from here:
https://dolphin-emu.org/download/