sounds like a memory management thing, so lets discuss over here shall we?
what carrier are you on? US or international?
have you rooted your device?
do you use live wallpapers? have a lot of folders/icons and lots of home screens?
all of these things eat up precious resources and if you see a lag when waking up the device, it could be an issue with memory management.
let me ask, prior to installing this app, did you see this behavior with the stock lock screen?