Question: I experience reoccurring incidents of `fontd` eating up all the process power. What could be the cause of this and how do I make it behave again?

Answer: 
`sudo atsutil databases -remove; atsutil server -shutdown`

From 50% cpu usage to 1%

Source: <http://icicity.com/2014/02/13/high-cpu-usage-caused-by-fontd-on-os-x>
You might want to check for defective fonts. I believe there are apps that can help with that. "Doctoring" is the term used often