UPDATE: iOS 8.1.1 released on November 17, 2014 includes "increased stability and performance improvements for iPad 2 and iPhone 4s."
iPad 2 owners may want to wait to upgrade to iOS 8. Apple's latest OS has been shown to run sluggish on both the iPad 2 and iPhone 4s. This should not be a surprise since the second-generation iPad was released in 2011.
Apple has been known to fix issues with newer firmware running on older hardware before, but has yet to address the iPad 2 or iPhone 4s, even after releasing two iOS 8 updates.
9to5Mac has confirmed that Apple plans to release three major follow-up versions to iOS 8 in 2015. Hopefully Apple will make the experience a little smoother and faster on its older hardware with one of its future updates.
This doesn't mean you shouldn't update to iOS 8. It just means the experience may not be as great on Apple's older devices and you may want to consider waiting.