Transfers from bank accounts are never instant as Paypal use electronic transfers ie echeques.
Funds normally leave your bank account from a few hours > few days later but they take several working days to clear through to the seller as echeque transfers are the cheapest (but slowest) transfer option and go via ACH in batches.
Paypals instant transfer isn't really instant, its called that because normally when you buy something paypal credit the seller upfront and then Paypal take the wait for the funds to transfer, this is so you get your item without delay.
There are times however when its you and the seller / recipient that take the wait for the echeque to clear and here are some of the reasons...
1. Back up card has expired or needs updating. 2. You have had a recent negative balance or failed bank transfer (so paypal are not going to credit the seller in advance in case it happens again). 3. For some high cost riskier payments / transactions. 4. Friends / family payments *funded from a bank account* as this option is free then its you that takes the wait this time.
Advice is voluntary. Kudos / Solution appreciated.