Most commonly refers to a person who is completely incapable of being ready to leave on time, or being anywhere on time. Can also be failure to go home once decided to leave, sticking around another hour or so after having said bye to everyone, having less than half an exam done with 5 minutes to go, anything of the like...
"Fuck's sake Jess hurry up, we're fuckin late as all shit!"
...turns to mate also waiting "My bad bro she has fuckin chronic lagging syndrome..."
..mate replies "Badly, no-one lags like her man!"