Someone reported this issue before - could be iOS limits app updates over data due to the size limitation. A workaround is to use a hotspot from another phone.

Bit hard to get anything more than 500 now, considering iOS 13 will not support iPhone 6

It's giving you a hint that it's time to upgrade

Try this

https://support.apple.com/en-my/iphone-6s-6...-issues-program

Do note it's for 6S and not 6 ya, as this thread is for iPhone 6/6 Plus

Don’t think it will make much of a difference, I think iOS 10 was the safety threshold for iPhone 6. I did find 12 to be a lot more bug-free then 11 back in the day, but even if it does help with the lag I would imagine it’d only be marginal.

Hahaha, key word is “owned”. Sold it long time ago

Since your phone is already on 11 and lagging, I submit that there’s probably no harm trying to update to 12. But that’s my view, probably check with current owners first!