How Visio slows down as more shapes are added

As more and more shapes are dropped on a Visio drawing the program gets slower, ok, this only becomes noticable with hundreds or thousands of shapes but here is an attempt to show this:

I created a Visio VBA macro that
  1. noted the time (a)
  2. dropped one shape at a time onto a page at a random location
  3. noted the time (b)
  4. saved the time difference (b-a) in a file (testdata.txt).
This file was then plotted in Excel (number of shapes so far against time in milliseconds). As can be seen from the graph below, the time to drop a shape initially is about 400mS but is near 1500mS at 300 shapes.

Visio slowdown curve