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.