IOS 7.1.2 very slow

I'm working on 2 projects with same issue.
When I run it on Iphone 4S with IOS 8 or above both apps are running smoothly.
But on iphone 4S with ios 7.1.2 is running poorly.
Is there any know issue about ios 7.1.2?
I would like know if there is any workaround about it.
Thank you for any help!