Does this directly affect your computer, or does it just happen in Osu? Do you play in fullscreen or do you play windowed? If it is windowed then you may be losing focus on the Osu window. Or does it actually only affect 4.5 star nomods? If the keys are just slow to register, play around with the setting a little. Enable the FPS counter, if it's under like 5ms, you're fine. I recommend disabling frame limiting.