Slow phone?

What do you mean with 'slow'?

1.

iPhone 6S is > 6 years old, 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, 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

2.

Check your internet connection.

If they are On, turn Off Hotspot & VPN.

3.

See -> Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch

Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support

4.

Try to free up iPhone device storage see this Apple articles below:

If you need more space for an update on your iPhone, iPad, or iPod touch

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

How to check the storage on your iPhone, iPad and iPod touch

How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

-> What's the difference between device storage and iCloud storage?

What's the difference between device storage and iCloud storage? - Apple Support