Release Preview Ring for Windows Insiders is for Windows Phone users who want to remain on the original Windows 10 Mobile Build but get the latest bug fixes and improvements (10586.xxx)
While the Fast and Slow Ring are for users who want the latest new features along with all the bugs and issues with it (14xxx)
Today, Microsoft released a new update 10586.338 for Windows 10 Mobile users on the Release Preview Ring, there are no changelogs from Microsoft, though it is safe to say it consists of more bug fixes and stability improvements.
There have been unconfirmed reports that this improves battery life.
I have updated to this latest update and after a few hours I can only confirmed user experience is a bit more snappier, but I have nothing to report on the battery life, seems about the same before the update.