iOS speed tracker app
http://abvio.com/runmeter/ will speak your stats:


Hear stats automatically at time or distance intervals, or on-demand with your earphone remote.


I've reported several bugs and enhancements to the developer and they have many features baked into the app and tuning of parameters if needed. I would buy their app on the assumption it can cycle that fast and contact the developer if you need a setting to get the level of detail you need. The engineering behind their use of GPS is excellent. You'll need their assistance in tuning the filtering to avoid false alerts when noise places you more than 100m from the last fix in a shorter period of time than you can cover on foot.