Is this on the Vox? I'm guessing you've tried rebooting it. If a reboot doesn't help, try uninstalling apps which have an active background presence. Hop into the Notifications menu and check what's running in the 'Ongoing' part of the Notifications screen. There might be something there chewing up CPU cycles.

It may also be worth noting that task killers in Android do pretty much nothing because of how Android uses RAM. Task killers generally don't help performance or battery life. They can actually negatively affect your performance and battery because of their constant background presence and monitoring of apps.