Hi Rich_Art - often times much of the viewport speed is taken up by the edge curve display - try turning off the hidden lines (On the side pane under View > "Display hidden lines") and see if that helps speed things up.

Also by default MoI generates a pretty dense surface display mesh which looks nice and smooth but can tend to bog down if you've got a heavy model with a lot of little curvy bits, like lots of fillets.

Try making the display mesh density to be coarser as described in this previous post and see if that helps:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4196.2

The settings for that are under Options > View > Meshing parameters - try setting "Mesh angle" to 25 degrees and uncheck "Add detail to inflections" and see if your display speed gets a boost.

- Michael