Some things to keep in mind when looking at core usage:
1. Sonar is not the only process running. Others will be using core 1 and maybe others.
2. Core 1 will normally have higher usage than others. That has nothing to do with Sonar. It's to be expected.
3. If you are using VSTs the way you configure them can affect core usage. For example, with EW Play it is best to use several instances rather than a single instance with instruments on separate channels.