It's not slower.

proprietary

Only the extension pack, the app itself is GPL.

not even as easy to install

How do you mean? You simply install virtualbox (and virtual-guest-iso if you want). All the relevant packages are available from official repos, at least on Manjaro, including a linux-virtualbox-host-modules-meta package that maintains the required linux*-virtualbox-host-modules as needed automatically.

Also, I prefer virtualbox because it makes a lot of stuff easy to do.

File sharing with the host for example, it's a matter of simply pointing at a dir in settings, as opposed to having to set up Samba on the host (which I don't have and don't want) and deal with wierd networking issues, or deal with virtiofs.

Don't even get me started on selecting locations for the vm files. Whatever you think it should work like, virt-manager's way ain't it.