When i wanna test m game i go make full screen then game start be so slow like they lag i dont know why i have to many assets on project or what i really dont know :/ what happen.
Can you Send ScreenShot Or Video, Or Check Events Again.
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There may be a million different reasons.
Run your project in Debug Mode (Shift+F4) and investigate yourself. Check the number of collision checks, number of objects, CPU and GPU profiler tabs.