If you've got a 6, upgrading it to iOS 12 will kill it.
iOS 12 ruined that phone.
At this point - theres no point in updating an iPhone 6 to iOS 12. iOS 12 won't provide any improvement and Safari will still break on most websites. iOS 12 is a poison pill for iPhone 6.
iOS 15 runs fine on a iPhone 6s though and I highly recommend upgrading that phone if that's what you've got.