Iphone 6s is lagging

If your iPhone 6S still has the original battery, it has most likely reached the end of its life. This could be the reason it's lagging.

Batteries degrade over time with usage due to chemical aging, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone.

This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Repair & Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

It’s probably because the phone is old and it’s somewhat outdated and can’t keep up with today’s standards I have an iPhone 6s and it does the same thing battery got replaced and everything time to put it away