By default, Jenkins is configured to run one "build executor" for each core in the server. This is a reasonable default for CPU-bound workloads (like most compile/test jobs), but it's a bit pessimistic in my case - CI pipelines for my Chef cookbooks.

Fortunately, the executor count can be increased with a small Groovy script - this thread on StackOverflow told me what I needed to know.

If you're already using the Jenkins cookbook to manage your Jenkins server, you can run two executors per core like so:

jenkins_script 'set number of executors' do
  command <<-EOF
import hudson.model.*

// Set executors to twice the number of CPU cores
Hudson hudson = Hudson.getInstance()
hudson.setNumExecutors(#{node['cpu']['total'] * 2})
hudson.setNodes(hudson.getNodes())
hudson.save()
  EOF
end