I feel that WP8.0 was significantly faster. WP8.1 with denim on my 928 is noticeably slower. Some apps take longer to open and resume (Family Room, Games hub, etc.) that it is very much so a bittersweet upgrade (because some things are nice, like Cortana and app folders, and other things are not nice, like the slower speed, all games listed in the apps section, and the notification center).