Hi makenzie165,
We understand you are experiencing a performance issue when using FaceTime.
We would recommend you Restart your iPhone.
If the issue persists, we would recommend disabling FaceTime, by going to: Settings > FaceTime, then Restart your iPhone, and re-enable.
If the issue persists further, we would recommend ensuring you have a backup of your data (How to back up your iPhone, iPad, and iPod touch), then Restore your iPhone. You can learn how to do so here: Restore your iPhone, iPad, or iPod to factory settings.
Once complete, Setup As a New Device (do not restore your backup).
If the issue is resolved, go ahead and restore your backup. You can learn how to do so here: Restore your iPhone, iPad, or iPod touch from a backup.
If the issue recurs once your backup is restored, there may be an issue with your user data, in which case we would recommend you sync your content to a service such as iCloud, then erase your device and Setup As a New Device - all synced data will return, though you will need to re-download your apps and adjust your user settings to your preference.
Hope this helps!
Thank you for using Apple Support Communities.
Cheers!