What takes so long for it to charge

iPhone 6S -> is nearly 7 years old, the hardware remains old, so does the battery.

Having the battery replaced by Apple or an Apple Authorized Service Providers may resolve the issue.

Replaced the battery of an iPhone 6S here last October (about $ 60.00), must charge every 2 days.

You should consider whether it is still worth replacing the battery.

iPhone 6S is also not eligible for the update to iOS 16, see iOS 16 Preview: iOS 16 Preview - Apple (UK)

(scroll down)

The battery can be replaced by Apple or an Apple Authorized Service Provider.

See this Apple article: iPhone Battery Repair & Replacement - Apple Support

The info page with the prices has changed, so -> Contact us -> Talk to an Apple Support Advisor

Or call Apple in your region see: Contact Apple for support and service

Contact Apple for support and service - 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

Hello ~ We would need more details about what exactly you are having an issue with but possibly your iPhone? If so your iPhone may need a new battery if health is below 80%…

Check the iPhone battery health and usage - Apple Support

~Katana-San~