Setting a build agent's priority

Given a pool of idle compatible build agents, how does TeamCity choose which one to use for the next build? The fastest one based on the CPU rank? Is there a setting that influences this choice?

Use case. Mix of 'always on' agents, and 'on demand' cloud agents. Agents are essentially identical. CPU ranks of agents are all within a few points of each other but vary as agents come and go. Sometimes the 'on demand' agents  come up with equal or higher CPU ranks than the 'always on' agents. This leads to them getting the new builds, and thus they do not shutdown, even though the 'always on' agents go idle.