Is there a way to "throttle" MacPorts?
You can set the number of cores used by updating buildmakejobs in the macports.conf file.  By default all cores are used. 


  buildmakejobs
  
  Number of simultaneous make jobs (commands) to use when building ports. Using “0” will cause a runtime autodetection to use all available processor cores.
  
  Default: 0


This file is in ${prefix}/etc/macports where ${prefix} default is /opt/local