Thanks for the detailed reply.
RAM does indeed seem to be needed (task manager doesn't really help here, showing very small values, but I estimate up to 2GB of RAM eaten by the 4 BOINC tasks in various project combinations). Fortunately I have 10GB so I'm good even with a lot of other stuff running on the system.

Can you please give me any links where I could start reading up on VBox projects optimizations? Perhaps there's a second 4x4x4 challenge in there somewhere :)