You can stop the recursivelyBlit recursion on any FrameMorph completely covered by a large opaque morph...

[davidedc]

...you could keep for example a list of the top n biggest opaque morphs (say, frames and rectangles) and check that case while you traverse the list.

Note that in fact you could stop recursion on any Morph which boundsIncludingChildren are completely covered, for that matter.