Runs perfectly fine on the 5.
What often makes us feel that our older phones are running slower with new OS' is that we do more with the new OS. When stuff like multi-tasking and other functions came along, we started using the phone more and doing more with it. This made it feel like it was slower because we do stuff that is more taxing on it. There were tons of complaints about battery life and lag with iOS 2. But people didn't remember that now instead of just browsing and sending emails and texts, they were now using their phone for hours more every day with apps like Facebook, Twitter, etc. That extra use and use of bigger and better apps are what caused the real issues. Just keep that in mind when updating each time. It's not always the update that causes it.