Same issue here, minus the crashing. I have a 4790k, and the BattleNet client is using 100% of it to update the PTR. Upon quitting BattleNet, it goes back down to idle levels.
I have a screenshot of my task manager + BattleNet window, but I can’t post links here, so just take my word for it 
Once it finished downloading the update, everything went back to normal. During the “Finalizing” process and onward, CPU went down to idle usage again. I’ll report back if another update seems to do this again.
(side note: I can’t write “Battle dot Net” without the forums complaining that I can’t post links)