Current versions of virt-manager (5.0) and gnome-boxes (48) don't enable hardware OpenGL acceleration through VirGL by default. Other software like VirtualBox might not even support VirGL. If Plasma detects that it's running inside a VM and VirGL is not enabled, it should display a warning about reduced performance and recommend to the user to run a KVM+QEMU+VirGL based VM software. While this might not solve the performance problem for users coming from Windows (but at least they know the cause), it could improve the impression of Plasma by users coming from other Linux desktops environment who want to try it safely in a VM first.
A while ago I wrote a boot service that is meant to detect this and warn the user https://invent.kde.org/sitter/plasma-minesweeper/-/tree/master/plasma-vm-3d-warning?ref_type=heads Didn't really do anything with it though.