Why is bitcoin-qt startup so slow compared to MultiBit Bitcoin-QT: Very long startup and at first launch needs to download the entire blockchain

Multibit: Starts up in a few seconds and downloads for only a few seconds

Why is Bitcoin-QT so slow?
Bitcoin-QT is slower than Multibit because Bitcoin-QT is a thick client(it downloads the entire blockchain,) whereas Multibit is a thin client(it does not download the full blockchain.)