Slow Multicasting

When I multicast 1 PC, the image comes down in 25 minutes. However, the
more PCs I try to image at a time, the slower it gets. I tried 5 PCs at
once, and the image came down in about 2 hours. Tried about 20 PCs and
after 2 hours, we were only 25% complete. Network ports are all set to
auto (I tried forcing speed/duplex too but no difference). The switch it's
all connected to shows no errors on the ports. I really do need to image
more than one at a time. Any ideas?