Frame monitoring system for gamers any solution?

An iPhone 6s is a ~seven year old design, very much resource- and performance- constrained, and is not supported by the current iOS 16. Its performance is and will be low.

You can certainly ask app developers to tune their apps for it, but I would not expect substantial improvements on an iPhone 6s even — with app tuning.

How much faster are newer iPhone models? Here is some iPhone performance data: your CPU is 608, current CPU is ~2504; your metal graphics ~2859, current metal is 22259: https://https://benchmarks.ul.com/3dmark-ios

You can probably compare performance with some existing tools: https://benchmarks.ul.com/3dmark-ios

Want better graphics performance? Replace the iPhone.

I'm not aware of a way to monitor frame rates. Not outside of app development.

iPhone and iPad frame rates can be variable, as well.

Here's some information on higher-end display frame rates:

... Optimize for variable refresh rate displays - WWDC21 - Videos - Apple Developer

The app developer might choose to display some of this information within a running app, but AFAIK it's not otherwise generally available.

If you're having issues with a particular app, check with the configurations supported by or recommended by the app developer, and check with the app developer.

... Contact a third-party vendor - Apple Support

You can send your feedback to the people that work for Apple here:

... Product Feedback - Apple