Why Sometimes FPS drops in Unreal Engine 5 while playing, especially in 4K, sometimes that can be a setting called Screen Percentage which can be different for viewport and Play In Editor (PIE), in this video we see this setting in detail and how to fix it.
https://dev.epicgames.com/community/learning/tutorials/ep8E/unreal-engine-why-fps-drops-in-ue5-editor-while-playing-and-fix