"Can't be benchmarked" – No.

Perhaps there is a way to benchmark such things: make a test 3D program which plays multimedia onto a rotating 3D cube, and outputs the frame rate and other latency data on the screen. Run this, then start up some other things that contend for the scheduler, like some I/O (copying a large file?), some network traffic (pinging a host in the LAN?), and such. See how the 3D app holds up under such strain, by watching the numbers.

I don't know how well this would work, but it'd be a test of some kind.