Purchased a book in Amazon, for Kindle; apparently Amazon has a web reader, great! However, it chews CPU (30% on two processes according to top), which is not great for battery life (also cooks your lap, haha).
So, to limit a cpu process to 5% max capacity of a single core:
rctl -a process:88673:pcpu:deny=5
where 88673 is the PID. So awesome that all of this is already setup / working out of the box!
To turn off a rule:
rctl -r process:88673:pcpu:deny=5
