I have commited a set of changes to _transforms, collections, quiver, contour, and numerix as part of a move toward taking advantage of the efficiency of numerix arrays in place of sequences of tuples. The changes are outlined very briefly in CHANGELOG and API_CHANGES.
Changes in clabel are hacks, and may make it less efficient rather than more, but I expect this to be temporary; I needed to simply make it work with the other changes.
Where breakage will occur is any place in user code that expects the collection segments or vertices to be lists of tuples and tries to append to the list, for example. I don't know of any way to make the move towards use of arrays without this problem cropping up; I hope it is considered tolerable.