Dans mon cas, it was because Hyper-V was enabled. I had it disabled in Windows Features but I was still getting the turtle icon in the bottom right of the VirtualBox guest window. To turn it off completely, I had to:
On the host computer, open Ligne de commande (as Administrator).
Enter la commande suivante:
bcdedit /set hypervisorlaunchtype off
Restart the host computer.
After this, there was no lag at all.
