iphone 6s slowing down cpu 's

This is the biggest complaint we hear and it's mostly from iPhone 6 and iPhone 6S users who are a few generations behind the iPhone 8 and iPhone X. Apple has admitted to slowing down their phones' CPUs if the batteries had degraded.
Of course, no one was pleased to hear that Apple was hamstringing their expensive device, and Apple tried to make amends by offering cheap battery replacements. You'll also see iOS 11.3 let you disable performance throttling. So there are two fixes for this issue.
how to fix this issue ?
change batteries and it will fixed or how to deal with it?