Is it true that bitcoin could be extremely slow?
In this case, if E wants to send some bitcoins to F, does he have to make a transaction all over again?


Not necessarily. A blockchain reorganization will cause transactions that were not confirmed in the "winning" blockchain to remain unconfirmed and placed back into the mempools of nodes which switched blockchains. So the transaction will still be in nodes' mempools and could still be picked up by a miner. If the fee was very low, the situation would be the same as having a "stuck" transaction.