CHR is short for Cloud Hosted Router.

It is a virtual router, running RouterOS. Images are available for VMware, Hyper-V, VirtualBox and generic x86 images for other virtualisation technologies or physical x86 servers.

VMware - vSphere - ESXi

You can use both x86 and the VMDK CHR image, but you get the VMware specific drivers for vmxnet3, pvscsi etc. in the VMDK. So you should use the VMDK and choose the vmxnet3 paravirtualized NIC to be able to use 10 gigabit interfaces.