Hi farman24,
Thank you for reaching out and posting in Apple Support Communities.
From what you are describing, one game app on your iPhone experiences lag when you play. Have you checked the App Store to see if there are any updates that need to be installed for the app? See, How to manually update apps on your Apple device - Apple Support.
Have you tried the suggestions found in If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support?
If the app is not developed by Apple, you will want to contact the app developer to let them know what you are experiencing. See, How to contact an app developer - Apple Support.
If the app is developed by Apple, you will want to reach out to Apple Support by going to Get Support.
Good luck!