Xen, The backbone of qubes, is One of the very few microkernels that is widely deployed. It's extremely efficient. It only does the minimum amount of work necessary to dispatch resources to different virtual machine guests
So comparing a VM running on a dedicated microkernal hypervisor like qubes, compared to QEMU or KVM which requires a monolithic kernel, it's going to be much more efficient.
But, when you start talking about the full desktop experience, with a window manager and mice and keyboards, and a guest VM, and a VM to run the desktop, and a VM to run the USB for the mice and keyboard, and a VM for the network stack, and a VM for the firewall..... It's less efficient compared to a system running a single QEMU VM with a monolithic kernel, and everything handled with a traditional monolithic operating system.
It depends on your use case, what you want to optimize for, quite frankly if you don't care about segmentation and security qubes is probably going to be too much friction for you.