Istaria will always use one core as much as possible. When people talk about FPS (frames per second), what they're really talking about is how many operations their computer can do per second. A faster processor will do more, while a slower processor will do less - Istaria will always try to do as much as possible, as a higher frame rate is usually considered "good".
A recent update to Istaria removed a throttle in the network layer so that network messages would be processed more quickly. For some players, they would see Istaria using more of the processor.
Players with a very fast processor but a video card that is proportionally slower might find that their CPU is never used 100%. This is because the choke point for performance is within the graphics system of their computer. The processor does some work for the rendering frame, then hands it off to the graphics card. If the graphics card is slower than the processor (enough slower), the CPU will sit around "waiting" for the graphics card to finish. This would result in the CPU looking under utilized. This situation is most common on laptops.
Please take note that most modern processors have multiple cores. The concept of a process using % CPU is an old concept from the days when computers had one processor (with only one core) and software was single threaded. When a program has work to perform on a core, it is always executed as fast as the processor can do the work. Because Istaria is a rendering program and mostly single threaded, it always has work to do - as doing more work means a higher frame rate. For a computer with 2 cores, Istaria will show 50% utilization (100% on one core and 0% on the other core). From microsecond to microsecond, work will be done on one or the other core, but almost never on both at the same time. The exception to this is during loading, which is multithreaded and could likely use all the cores available.