Performance issue

[bertfrees]

The px:mediaoverlay-rearrange test in mediaoverlay-utils takes a lot of time, so much that Travis thinks the build has stalled (Travis stops after not having received any output for 10 minutes). Debug messages show that it's the XProcSpec part that takes so long. More specifically the "grouping x:expect elements with their x:context..." and "reporting..." seem to take the most time.