Hi EmilTee,
If I understand correctly you only want to backup your slow motion videos to iCloud when it backs up. When you go here:
- Tap Settings > iCloud.
- If you're using iOS 8 or later, tap Storage > Manage Storage.
- If you're using an earlier version of iOS, tap Storage & Backup > Manage Storage.
- Tap the name of your iOS device.
- Under Backup Options, turn off any apps that you don't want to back up. When you're asked to confirm, choose Turn Off & Delete to turn off Backup for that app and remove all of its data from iCloud.
Manage your iCloud storage - Apple Support
The option for Photo Library only has a single toggle, so it looks like it is all or nothing, so to speak. You can however import the photos and videos you have to a computer and delete them from the phone to reduce the size of the backup:
Import photos and videos from your iPhone, iPad, or iPod touch to your Mac or Windows PC
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