I can't repeat this today! This follows a PC reboot, although otherwise everything else looks pretty much the same. I tried that same 4x4 collage and half a dozen others, including Magix collages, and I'm delighted to see that all now play back smoothly.
I'm guessing some temporary glitch in MEP 2013 that wasn't fixed by a restart, or some resource usage problem that didn't make itself apparent in other apps yesterday, suppose I'll never know.