I've tried about five images so far. The fastest was debian sarge with no X (guis). i ran it with:-
Code:
qemu -net none /path/to/image.img -m 512
dsl.iso (damn small linux) came second for me also with 512mb of ram. Fedora and centos c-r-a-w-l-e-d.
However I may not be comparing like with like here, just my mileage.