Place the following in your `Vagrantfile`.
```
Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
config.vm.provider "virtualbox" do |vb|
vb.memory = 1024
vb.cpus = 2
vb.customize ["modifyvm", :id, "--cpuexecutioncap", "40"]
end
end
```
Run `vagrant reload` to propagate changes.