If you are also frustrated by the fact that by default, Parallels will only write the RAM image to disk at 20MB/s even on a fast internal drive, then fear not.

Go to Virtual Machine > Configure > Hardware > Boot Order and add this to the Bootflags:

vm.mem_flush_limit=0
vm.mem_anonymous=1

I now get 40MB/s write speeds and it takes about 8 seconds to save a 2GB image on my MacBook Pro, due to the use of compression I presume.