I have noticed that after a few minutes of becoming online, and randomly perhaps a couple of times every hour (??) tor uses 100 % processor on all cores for about 40/50 seconds.

I am curious what is tor doing internally to need so much power? There is normally isn't more than 4 mbits going through my relay, I noticed this a while back on htop before the December load increase and on previous versions so this has got to be an "internal thing". There is no log entry when this happens.

I do not think it is a bug I am just curious why it needs this much computational power, 

Thanks.