Battery
iPhone slows when the battery is down by 20%
iPhone 6s, iOS 15
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iPhone 6S is > 6 years old (September 9, 2015), which also affects the battery life. That may be the cause of the problem.
Having the battery replaced by Apple or an Apple Authorized Service Providers may resolve the issue.
You should consider whether it is still worth replacing the battery.
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, 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
See also -> The last update killed my battery!
The last update killed my battery! - Apple Community