The network file system to host is usually pretty slow no? That was my impression.

As for memory ballooning, the main issue with it is that it (generally) only gets triggered when the host runs out of memory.

For a host which is only running VMs, this is fine. But for the typical consumer host it becomes cumbersome as you still need to give the VM a giant memory block and hope that your VM of choice is good enough to free on time. It's also uncoordinated. When swapping needs to happen, if the VM was using the host for allocation the host could much more efficiently decide what needs to go into swap.

And if the host was in charge of both the memory and file system, then things like a system cache could be done more efficiently on top of all that.

> The network file system to host is usually pretty slow no? That was my impression.

NFS doesn't have to be slow. If you avoid traversing the TCP/IP stack, performance is fine. Linux guests can use vsock to communicate with the hypervisor directly, and macOS hosts can use the Virtualization framework to map a guest vsock to a host UNIX socket.