2 ivan and Doug: Regarding the VBox speed, I see winxp guest is reasonably fast (I even thought, it flies for me). Also, as rklipp notes, we're working on VT-x/AMD-V support, and with it, it is even quicker. Regarding the Additions ISO, you just need to choose Devices->Attach the guest OS Additions ISO image, then agree to download it. It take some time to download it. Also, you can download it manually from
http://download.virtualbox.org/virtualbox/5.0.6/VBoxGuestAdditions_5.0.6.iso (the link to Addition ISO is also on the VBox Netlabs page at
http://trac.netlabs.org/vbox/, near the last release link). And then you can put it in the VBox main directory. It should be called VBoxGuestAdditions.iso to be found. (Upd: It downloads it somewhere, but not to the program dir, and in the end the VM crashes. Need to fix that.)
PS: USB 2.0 and 3.0 is available only in commercial version (they require a commercial extpack to be installed, which is not available for OS/2, yet (maybe, in future)). USB 1.0 (OHCI) should work, but raw USB device passthrough is not ready, yet. But you can attach USB mouse or USB harddisk to a VM (you only need to add an USB controller to Storage section, and attach a .vdi to it).
PPS: Also, no need to disable sound, as it uses Null audio driver. So, guests will see the audio card, but the sound will be not output. It should not affect the performance.