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Bitcoin Core now uses 'headers-first synchronization'. This means that we first
ask peers for block headers (a total of 27 megabytes, as of December 2014) and
validate those. In a second stage, when the headers have been discovered, we
download the blocks. However, as we already know about the whole chain in
advance, the blocks can be downloaded in parallel from all available peers.
In practice, this means a much faster and more robust synchronization. On
recent hardware with a decent network link, it can be as little as 3 hours
for an initial full synchronization. You may notice a slower progress in the
very first few minutes, when headers are still being fetched and verified, but
it should gain speed afterwards.