KDE mirrors release xdelta files - but portage does not use them.
If you dont know what xdelta-files are - these are much similar like diff-patches but for binary files.
So downloading xdeltas reduce the size of your downloads.
For example instead of downloading ~300 MB for the complete kde-sources (incl. kde-i18n-3.2.1.tar.bz2) you would download only ~50 MB xdelta-files.
I have written a small script, that does this for you now.
You can find the script here:
http://linux01.gwdg.de/~nlissne/kdedelta.sh
Have a look into the script before starting it
(for example you can/should change the packages and the kde-mirror to be used for download...)
The script checks if you have the previous versions (referred as $KDE_OLD and $ARTS_OLD in the script) and if so it will download the xdelta file and use it together with the old version of the tar.bz2-file to create the new one.
If you dont have the old version in your $DISTDIR it just downloads the new version.
Just start it before upgrading your KDE.
Of course you should have installed xdelta to use it. (just emerge xdelta)
Script is prepared to download xdelta-files for arts-1.2.1 and kde-3.2.1 but can easily be changed for future versions.
I hope this is useful for someone.
Have fun.