We have 40 agents, 1/2 are fast 1/2 are slow. When there are both slow and fast agents available with the right capabilities to run a job, we want Bamboo to pick the fast ones first. The job should only run on slow ones if all the fast ones are busy.

Assigning a priority to agents (or a metric like for a nic gateway) would be one way to implement this. 

If I have 3 levels of PC performance, I would assign 1 to the fastest, 2 to the medium, 3 to the slowest.

Jobs only go to 3's if no 1's or 2's are free to run the jobs. They onlyi go to 2's if there are no 1's free to run the job.