What is the fastest way to back up my iPhone?

For most users, connect the phone to your Mac or PC using the cable. Not only is this usually the fastest way to back up, it is also the most reliable way to back up.

The downside.....not the most convenient way to back up.

Back up iPhone - Apple Support

Personally, I prefer to back up using the iCloud method. It’s easy and I have it set up to back up automatically so it involves no work on my part once it is set up.

How to back up your iPhone or iPad with iCloud - Apple Support