When we're given input-only iterators, this repeatedly mid-inserts, shifting down existing elements to make room. That's O((last - first) * (vb.end() - where)), which is forbidden by WG21-N4910 [vector.modifiers]/1:

Complexity: If reallocation happens, linear in the number of elements of the resulting vector; otherwise, linear in the number of elements inserted plus the distance to the end of the vector.

Which applies to vector<bool> too, [vector.bool]/2:

Unless described below, all operations have the same requirements and semantics as the primary vector template, except that operations dealing with the bool value type map to bit values in the container storage and allocator_traits::construct (20.2.8.3) is not used to construct these values.

It's extra horrible because each insertion is performing a ton of bitwise work to shift the packed elements down by 1 bit.

Will also affect vector<bool>::insert_range(where, range) after merging #2806.

Note that vector<T> is unaffected because it appends followed by rotating the elements into place: