Well, IMO, your first problem is that you are using the wrong software to construct/edit a diagram. Instead of using a Word Processor you should be using software designed for that purpose (i.e., PowerPoint, MS Visio, et al). You indicate that you are using MS Office 2010 and MS PowerPoint 2010 is part of that software package. You could benefit greatly by using that program to construct the diagram and then insert the completed diagram into your MS Word file as a .jpeg or similar file.

Of course, the size of the file will vary by the content so, you could end up with a similar problem (running slow) if it is too large. However, not knowing what your situation is...I am just guessing.

Hope this helps.