I would suggest a hard reset because if this started after the Denim OTA there was probably a glitch during the update.
I presume you already tried a soft reset and that you did not install an app which could case this or added for example Cortana as a tile to your start screen.
I think the update was fine, however multitasking is very slow like for example switching between apps takes a lot of time.
I'm little conscious about my phone during the hard reset process.