Unfortuantely composer requires a lot of RAM & processing power. Here are a few things that I did, which combined, made the process bearable. This was on my cloud playpen env.
service mysql stop
(kill your DB/mem-hog services to free some RAM - don't forget to start it again!) top
to watch memory/swap consumption until process is complete.composer.phar update --prefer-dist -vvv
(verbose output [still hangs at some points when working] and use distro zip files). Maybe try a --dry-run
too?