Why with use less 'CPU'; of course. Seeing how that's currently unimplemented (but it could be windows -- hmm, this should be a wishlist item for PAR), you'll have to do it like

cat foowithlesscpu.bat
cmd.exe /c start /LOW foo.exe
[download]

or write it in perl with Win32::Process.

I should note that it should only be CPU intensive only on the 1st run when par has to extract all the modules ... after that they're cached (then it's as cpu intensive as it takes encode the message).