If you want to build ports on a machine, you should add a large (several dozen GB at least) /usr/ports partition. Keep in mind that monsters like firefox and libreoffice have insane space requirements.
Diverting ports builds to /home is a hack that works around the wrong choice made during install (which will invariably happen when you first start out, that's ok -- be prepared to reinstall with better parameters once you learn more about what you need).
I have built Iceape on gNewSense linux on an X60 and I can say that under linux you need at least 20Gb of space and 6 hours on the core-duo with 2Gb RAM.
I always plan on a throw-away install when first playing with an operating system and repeat the install when I know what the 'rules' are. I shall be encrypting my /home just for peace of mind if I leave the laptop on the bus, so that is another thing to research.
Diverting ports builds to /home is a hack that works around the wrong choice made during install (which will invariably happen when you first start out, that's ok -- be prepared to reinstall with better parameters once you learn more about what you need).