Weird bmesh performance when creating new vertices

I have 2 different meshes with roughtly the same amount of faces/verts. In the first case I only have 1 object in blend file, in second case around 80. This is the only difference I can think of, in both cases verts don’t have any layers associated with them, and no other things I can think of.
I’m running a script on both, that add roughly the same amount of vertices. I was measuring only the calls to mesh.verts.new(), and the results were quite odd. In the first case creating roughly 200k verts took 3.5 seconds, in second case 45.5 seconds.
So the question is, what can cause such a huge difference in performance when doing something as basic as adding a vertex?