[JKT]

I just ran an experiment while downloading OmniOutliner Pro 3.6 and experienced the same thing as you... OmniWeb maxes the CPU while downloading if the Download Manager is open. I also ran a few tests and the hogging also occurs when the manager is closed or minimised to the Dock as well; however, if you pause the download, it stops and only starts maxing CPU when you resume.

IOW, this is happening for me too.