I assume that you are attempting to get the development code, rather than the last release (2.0.0) which has 'tars' available here:
There is a 'download snapshot' button on the 'Code' page, however that appears to download everything (2.4GByte) even if you navigate into a particular directory.
My suggestion would be to use a Subversion (SVN) client, even if you don't intend to do any development. By default SVN only pulls down the latest release, not the whole archive.
You can extend the path to 'cherry pick' specific directories that you are interested in. For example 'svn checkout
svn://svn.code.sf.net/p/speed-dreams/co ... s/karting/ karting' would only pull down the karting tracks.
SVN will handle compression on the download, and you could then zip/tar it up if that whats you really need.
Simon.