Unfortunately the pluign documentation is a bit short and should cover more use cases. One of them is the assignment of a default label and, in case all hosts with that label are busy, a fallback label so that the job only has to wait if all hosts with both labels are busy. This would allow to specify a preference for faster hosts and fall back to slower ones if all fast hosts are busy.

Here's what I've tried so far:

import jenkins.model.Jenkins

Jenkins.instance.getLabel('FastHost').getNodes().each { agent ->
  def executors = agent.getComputer().numExecutors
  def busy = agent.getComputer().countBusy()
  if (busy < executors) {
    return "FastHost"
  }
}
return "SlowHost"

But this always returns "SlowHost", no matter how many executors are free on "FastHost" hosts.