How is your iPhone performance

I’m not sure what you mean. You should update iOS as soon as a new version is available, because every update potentially fixes security vulnerabilities.

If you mean how often should you get a new phone, that answer is personal. If your current phone meets your needs, just keep it. If you hear about a new feature that is important to you, get a model that has it. There are quite a few users who visit the forum who have an 8 year old iPhone 6; they should probably replace it, because it can’t run most newer apps. But a 6S can.

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.