High CPU load of Windows 10 VM in Gnome Boxes


I have (well, had) two Fedora installation - Workstation (PC - AMD Ryzen™ 5 5600X × 12, 16GB RAM) and Silverblue (Lenovo T495, all AMD, 16 GB RAM). I’ve created Windows 10 VM in Gnome Boxes in both of machines. With same VM setup (same amount of vCPU, vRAM and storage), with guest drivers and addons installed the Windows 10 VM in Silverblue install on the laptop was so slow I could barely use it. vCPU was all the time busy 30-40% and more. I waited until updates were installed and Windows search indexing was over, but nothing changed - e.g., it still took 7-10 seconds for Firefox to start in Windows VM. Internet search didn’t help, thus I installed Fedora Workstation to my T495 as well in hope that FW is somehow technically different. And It has helped - now I can use Windows VM on the laptop normally.
I’m wondering what Silverblue specifics could result to such significant impact to Windows VM’s performance. I’d prefer Silverblue on my laptop, is there anything I could do to fix that high vCPU load ? Did you experience anything similar ? Have you fixed it ? Would you mind to share ideas/solutions ?


Maybe the filessystems are different? When using btrfs with big files which change occasionaly you have to disable CoW. virt-manager does it, I’d assume gnome-boxes did too.

Generally speaking (as in: doesn’t seem to be the case here, but I’ll type this in for posterity), I use virt-manager and apply these Hyper-V englightments. At the bottom it mentions the clock enlightments reduce idle CPU usage.

On both machines I’m using EXT4 for host filesystem. In Windows guest, e.g., Outlook took majority of that 30-40% of CPU load. I wasn’t able to identify what caused that CPU overhead. I even downloaded and installed latest stable KVM guest drivers and addons expecting that it had some fixes, but there was no significant impact to CPU load. Essentially same quest setup on same laptop but with Fedora Workstation was completely different story. I’m wondering what’s so different between Silverblue and Workstation apart of immutable OS features of the former.