FB's data structures are hugely messy and decentralized. There's so much information that even FB can't reliably delete all of it for an individual user. Instea

FB's data structures are hugely messy and decentralized. There's so much information that even FB can't reliably delete all of it for an individual user. Instead, they erase or hash the IDs that represent you in the system, insuring that the links between your data and the front-end are broken. The jobs that do this are scheduled, and every time you log in you create new IDs and nodes in the cache, which means they have to rerun the processes that erase the data. The delays are probably related to that.