Bitcoin-qt, OS X Mountain Lion, blockchains, and bootstrap.dat
Bitcoin-qt verifies the entire blockchain the first time it starts, to be sure that all the transactions are properly signed, and all the blocks certify an appropriate amount of computation (proof-of-work).  This ensures, for instance, that nobody is feeding you a fraudulent bootstrap.dat.  Depending on the speed of your computer and network, this verification might be the bottleneck, instead of the download.

There is not really any way around this if you want total confidence in your copy of the blockchain.  The bitcoin-qt client generally is designed for greater security at the cost of performance.  There are other "thin" clients that are less paranoid, and are willing to trust the blockchain they get to a greater extent, but there is a security tradeoff.  Whether you think the tradeoff is acceptable is up to you; many people do.