iOS 14 make my iPhone X perform bad

You implied that your health is below 80 percent. If your battery health is below 80 percent, it's time for a new battery. That's not a result of the update, just general degradation of the battery. Visit https://support.apple.com/iphone/repair/service/battery-power

Have Appple Store fit a new battery if it is 79% health

Not absurd. Your battery has degraded. It happens after 500 or more charge cycles. Your updated iOS is now showing more accurate reading.

You can get a fake battery fitted for about 75% cost of genuine if you want to take the absurd risk.

No. The volunteers that participate on this site are Apple product users just like you. We do so of our own volition, and the vast majority of  participants want nothing more than to help solve problems simply because we enjoy it