As BitTorrent is a swarming technology, more peers usually means faster downloads so finding as many as possible is a good start to getting a quick transfer.
Just as information about available peers sharing particular content can be discovered from a BitTorrent tracker, Peer Exchange is an additional feature which allows peers to communicate with each other directly, to discover if there are any additional peers that one peer knows about, but that others do not.
As far as efficiency is concerned, this communication takes place directly with other peers and as a result, some load is eased from the torrent tracker.
Although it already supported the basic protocol, up to now the torrent engine inside the Opera browser didn’t support Peer Exchange. The introduction of Opera v9.50 Alpha 1 (Kestrel) changes that situation and sees the introduction of a uTorrent and libtorrent compatible feature. As is usual with any uTorrent compatible peer exchange, peer exchange with Azureus clients will still not be possible until Azureus makes some changes to their client.
The Windows version changelog lists a whole bunch of bug fixes and tweaks.
For those wishing to take the new browser for a test drive, the advice on the Opera site is to do a clean install and “copy over your profile data (but not your preferences file) if you’d like to test the upgrade procedure. You may not be able to downgrade if you upgrade an existing install!”