Hello, I just wanted you to know that i have solved the problems with my HTC 8X both lagg, locking and overall performance after updating to 8.1 and the Cortana release as well by just doing a hard reset. It was not easy as the phone was not even starting up, was getting in blacks blacout after the HTC white splash screen and the hard reset combination of Volume UP, Down, Power and Down combination did not work, it just booted. So i had to deplete the battery, after that somehow the lack of energy did an actual reset and it let into the OS again, still with flaws but at least I could manage to do hard reset via OS (settings). After that everything was easy as butter. Please note that after doing the hard reset it comes alive again with Windows Phone 8.1 if you already did the update but it gives you the option to do a recovery of your apps and settings if you had that option enabled before. This is my best recommendation. It now works as a charm and now I even have Cortana enabled as the hard reset allowed me to select English US as my native primary language and all the the rest of the region and keyboard settings. If you need any help please let me know.