Why nested virtualization for the Android emulator is very very slow?

Nested KVM virtualization should have a small overhead, but an Android emulator inside a VM should be usable.

When I launch the Android emulator inside ubuntu 20.04 guest (on ubuntu 20.04 host), it warns me that I'm on a nested virtualization and thus it will be slow. But it's painfully low, not just slow. It takes 10 minutes to boot into Android and yes, all the systems are x86_64, even the android image.

I thought this had to do with OpenGL so I enabled OpenGL virtualization on my virt-manager and thigns are still slow.

I tried the same thing on my Ryzen 7 2700x which has 16 cores, I gave all cores to the VM and passed an AMD gpu to it, and Android emulator is also painfully slow.

Why?

from Why nested virtualization for the Android emulator is very very slow?