Apple released iOS 8.1.1 on Monday. The patch notes for the release specify just one thing: "increased stability and performance improvements for iPad 2 and iPhone 4s." The released also includes unspecified bug fixes.
Apple's patch notes in full:
iOS 8.1.1
This release includes bug fixes, increased stability and performance improvements for iPad 2 and iPhone 4s.
For information on the security content of this update, please visit this website:
http://support.apple.com/kb/HT1222
The update is a 1.75GB download through iTunes for an iPhone 6. Your mileage may vary by device.