I've used the free version of Cyclemeter for 5+ years, and it works well on my old iPhone5s (don't think the app has been updated for several years). I tried Strava and found it too complicated; tried another popular free cycling app and Cyclemeter does more and is easier to use.
As a speedometer in the proper sense, where you see real-time speed, yes it works like that too if you strap it to your bar and turn off the "auto-off" feature on your phone. It does wear down the battery fast, though; 33 miles this morning, and I went from ~98% charge to 66% at the end of the ride. The speed/distance display is large and easy to read, at least on an iPhone, and at least for whatever version of the app is the latest for the iPhone 5.