Hello feflrj.
Welcome to Apple Support Communities. We understand that you want to restore your iPhone from a previous backup. We’re happy to help.
Restoring your iPhone removes the content that is on your phone. It will not delete the content you backed up to iCloud.
We recommend creating a backup prior to restoring our phone. Here’s how to backup to iCloud: How to back up your iPhone, iPad, and iPod touch - Apple Support
Please use the steps here to restore from your iCloud backup: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
You mentioned your iPhone has slow performance. Please try the steps here: iPhone Battery and Performance - Apple Support
If you haven’t done so, please use the steps here to ensure you have the latest iOS update installed: Update your iPhone, iPad, or iPod touch - Apple Support
Let us know the results.
Take care.