Update: 6/21/2015 - 3:43 AM PST - Added third step that may resolve certain issues stemming from prior iOS backups restoring to iOS 9 Beta 1.
OK we've done our 24 hours of testing and can very safely conclude that all battery life issues accross three different devices included in the test have been entirely resolved and now the devices are not heating up, charging normally, and retaining a normal rate of discharge on standby. In additio, battery use reports within Settings confirm that the Homescreen no longer is consuming a substantial (more than 20%) portion of the average use of total battery power. (devices are normalizing data still, however to ensure none of you suffer this any further here are the necessary changes)
1. Completely disable iCloud Backups on each device running iOS 9 Beta 1 (Use iTunes to Backup and restore devices for now.)
2. Disable iCloud drive (if you enabled it) on each device running Beta 1
3. If you restored a backup from iOS 8.4, 8.3 or earlier after upgrading to 9.0 Beta 1 then you may wish to perform a reinstall of iOS 9.0. Simply do this by using the "update" versus restore function in iTunes. Do not restore any backups. (Assuming you did not reset all data and settings prior to doing the update then all your data will still be there.) - There were some devices in our lab that did behave differently regarding battery use and background operations. This had a positive effect, making all devices in the lab perform equally.
- Please bear in mind that the performance expectations on a 1st beta release should be minimal, however we were seeing iPads drain in a matter of a few hours on stand-by with some iPhone 6 Pluses doing the same while others did not. (Each had a slightly different variation of applications loaded, but all had the same iCloud account attached to them.)
There appears to be a SERIOUS problem with the API handling backups to iCloud or some sort of permissions issue with the data being written to iCloud. This likely went into effect during the update phase that indicated "iCloud data is being updated". (During the first Beta 1 load.)
This has confirmed to entirely resolve the "somewhat unpredictable" and extremely high rate of burn on the battery for this beta on iPhone 6, iPhone 6 Plus and iPadf Air 2. There is no reason to believe other affected devices will respond any differently to these changes, so we encourage other developers to try them asap. Apple should likely address this with the next update, when we can resume use of iCloud as normal during testing.
NOTE: iCloud data used by Third Party applications and other data like Calendar, Contacts and so on, does not appear to be causing any issues with battery usage, nor does there appear to be any data corruption. However do proceed with caution if you are using iCloud for your beta application development.