gangopersand:
look what processes you are running.
In your picture the numbers of the memory usage seem somewhat strange to me. Normally, the sum of available memory and system caché is below of total memory. And more, the system cache in your case is very,very high and almost reaches the total system memory.
This could mean, that what is slowing down your machine is swapping (virtual memory <--> disk). Also, if I'm right, the spike visible in cpu load could be the start of the download, and just after memory page file starts to work.
Try stopping some stuff you are running, then retry testing with hfs.