If you have a slow connection and machine, you can still try to build ROMs from source using the Google Cloud platform. This guide by XDA Senior Member nitin.chobhe walks you through in using the free trial of Google Cloud to achieve the same!
See 1st page: http://forum.xda-developers.com/chef-central/android/guide-how-to-build-rom-google-cloud-t3360430
And 2nd page: http://forum.xda-developers.com/showpost.php?p=54118614&postcount=2