IOS 7 worked fine on mine.
When it first came out, it was sluggish in some programs. Such as zinio. But this cleaned up over time, as programs were upgraded to support retina.
But I've been able to get ahead of it on several occasions today. And noticing that several programs weren't running right. They may have to be upgraded.
I deleted these programs, hoping that a fresh install with clean data would work better. But I think Apple's servers were bombarded with IOS 8 upgraded a couple days ago, and all the new iPhones connecting. THe programs wouldn't download.