I think it might be a battery saving feature that is not adjustable in metro / modern mode.
However, try desk top version of internet explorer. Lots of people use that for spotify etc. Might work better for background.
Or get a 3rd party YouTube app and then put it on part of the split screen (multitasking)
I think the ARM and low power features of surface rt make it run better if only one tab is chugging at the same time. I notice the same thing happens on my nexus 4. And same thing happened on my iPhone 3gs... Which I had to hack extensively to multitask.