LR often disappears from Task Managers' Processes tab, especially when it's grabbing all the cpu time. Wondering why. (Thought I saw it listed under the Services tab last night, but it was sorta late...)
However, during an export, LR will suck up all the available CPU it can. If I know I am going to be doing a large export, I will drop the priority on LR to "below normal" so that the system remains responsive while LR still takes all the CPU left over from everything else.