I don't think speed is a matter of your container engine (Docker vs Vagrant) but what virtual machine you are using. I usually use Docker without any virtualization since I'm running linux containers on linux. I'm guessing when you say Vagrant, your're probably using VMware or something. I don't know what Docker on mac might be using since I avoid using mac.
Yeah, so it's Virtual Box using your resources, not Vagrant or Docker. You might want to read up a bit on how containers work -- on a unix system, assuming same cpu architechture (so you don't need a VM) and filesystem that supports overlays, containers have barely any overhead in terms of system resources. You could have a thousand containers running on your machine and be fine. VMs, on the other hand, have pretty serious overhead.
