Need a fix for 15.4
New update on ios 15.4 has given lot of issues lag and battery dropped etc
iPhone 6s
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iPhone 6S is > 6 years old, having the battery replaced by Apple or an Apple Authorized Service Providers may resolve the issue.
Have an iPhone 6S here, and had the battery replaced by an Apple Authorized Service Providers (about 5 months ago), because it is still possible.
The battery can be replaced by Apple or an Apple Authorized Service Provider.
See this Apple article: iPhone Battery Replacement - Official Apple Support
If you are going to replace the battery, by Apple or an Apple Authorized Service Providers read this Apple article:
Get your iPhone, iPad, or iPod touch ready for service
Get your iPhone, iPad, or iPod touch ready for service - Apple Support