Dropbox is always posting random betas and new features in its user forums. The latest beta, as pointed out by Ghacks, posted just last week speeds up syncing for large files for both shared folders, and any of your computers synced with the same Dropbox account.
In the past, Dropbox would wait until a file was completely uploaded to its servers before allowing any connected accounts to begin downloading said file. With the latest beta, users running version 2.9 and up can begin downloading the file the moment it starts uploading. As you can imagine, this speeds up the sync process tremendously.
At least, that's all in theory. This is a beta and not quite ready for a public release, so it's possible you'll run into an issue here and there. Be sure to report it in the forum if that's the case.
You can download version 2.9 of the desktop app for OS X, Windows, and Linux from this forum post. You won't need to enable anything in settings for this to work, it's simply part of the update. Happy syncing!