Inserted images cause Word 2007 to operate extremely slowly

From time to time, I develop Word templates which contain large image files
placed into the header to provide a stylised background or company logo. I
have been developing these in Word 2003 for years with no problems, but now
with 2007 I have noticed that placing an image causes the file to operate
extremely slowly - ie. there is an long lag when typing.

I usually insert JPG format images at 200dpi or 300dpi - I have noticed a
small improvement if I reduce the image to 150dpi but still not acceptable. I
have just been experimenting with different file formats (TIF, GIF, PNG, BMP,
EPS, EMF, WMF) all exported from Illustrator and it seems that any raster
based format causes this lag. I had some improvement using EPS, EMF and WMF,
however these formats all seem to be quite a messy looking export from
Illustrator.

Any suggestions for improving speed performance using raster files, or
alternatively for getting better quality EPS, EMF or WMF exports?