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When I'm not running the game, I can have my CPU at a constant 2.4 GHz. But for some strange odd reason, whenever I start up XIV, it drops to 800 MHz across all 4 cores. I have no idea what is causing this. It's extremely frustrating since I know it's a big cause of my low fps :/
Sounds a bit like your CPU might be overheating, and to prevent any damage it lowers the clocks. Try checking your temperatures with RealTemp or Speedfan, and if they seem high you might want to consider getting a better cooler. "High" is very model specific though, but I would consider them high if they are over 50-60C on desktop or near/over 80C while doing something like gaming.
Can you tell what model is your CPU? You can use CPU-Z for example to check the model. That helps in troubleshooting your problem, there might be some 'feature' (powersaving etc) that kicks on.