iphone 6 jailbreak or optimize?

I would say it’s never better to jailbreak, and doing so is not supported nor recommended by Apple. You open yourself to all sorts of potential malicious code and exploits that simply cannot and do not affect a native iOS installation. That is especially so if afterwards you start installing apps from anonymous web sites or unknown developers or sources (such installations can inherently install all sorts of code that are not disclosed to you and which you have no awareness of - it’s no different than granting root or admin access over the internet for your computer to someone you don’t know, allowing them to do things you have no way of knowing about).

Also using a hack to gain hotspot capability when your cellular account or provider do not allow for such can possibly lead to issues with your carrier, like having your account throttled for unexplained “excessive” bandwidth use.

Just replace the aging battery for $29 at an Apple Store.

Settings / Battery / Battery Health to learn more.

Did you update to iOS 11.3? If you did, you can turn off the throttling that was installed to protect your phone from shutting down.

Upgrad to iOS 11.3. There is totally new battery management settings in iOS 11.3 and you can yourself turn off the forced throttling of older iPhones, which may fix this for you.