All Guests VERY slow on Ubuntu 12.04 64-bit Host (seen in Virtualbox 4.2 and 4.1)

[Lachesis]

I've had extreme slowness on a variety of VirtualBox guests in several versions (at least 4.2 and 4.1) on my Ubuntu 12.04 (also seen on 11.10) server in VBoxHeadless. It's so slow that even booting Arch Linux (guest) takes more than ten minutes (instead of 30 seconds normally). I've got plenty of RAM (8GB of 12GB free right now). The weirdest thing is that restarting the host seems to fix it for a few hours or days. Nothing else helps, not even unloading and reloading all the modules.

Currently running VirtualBox version: 4.2.0_OSEr43316

Host: Linux tron 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

Host dmesg and Arch Linux Guest 32-bit and 64-bit logs attached. I've seen the problem in Windows Server 2008 as well.

[Frank Mehnert]

Does the same happen if you enable the host I/O cache for the disk controller in the VM settings?