So, this is weird. I had my battlenet agent set to throttle background (future patch data) downloads to 100kb/s (intentionally slow, yes). During downloads the agent.exe process would peg a CPU core. I only noticed because my case fan spun up. Oddly, if I remove the throttle (set it to 0) the problem went away. HotS was downloading a small future update in the background when I noticed this.
I can reproduce this downloading “foreground” apps too. Currently, downloading Hearthstone update and with throttle set to 500k (“latest updates” setting) and it also pegs a core. Removing throttle makes it quiet as well: CPU goes to a couple percent.
I think you guys have a performance bug in the download throttling logic. Did someone write a busy wait loop to throttle the download?