The more the CPU, the faster ES is. So using CPU cycles is perfect, and 85% is nothing! This means system load is less than 1.0. (Note that my bulk sessions can rise up to 8.0 -12.0 system load)
You can look at your client and decrease bulk request length and bulk request frequency, you can even implement pausing between bulk requests. This is very easy and will slow down indexing.
If you want advanced configuration, in 1.5.2 you can use store throttling to slow down ES nodes, even if clients do not want that.
The motivation was to solve issues when slow disks block CPU on ES servers. On modern hardware, this is no longer the case. Be aware store throttling is removed in ES 2.x, it was an advanced setting, and must be handled carefully. For example, all nodes should throttle at same rate.