Hi,
First of an 100 page XFA Form is pushing it. In particular when the form is opened in Reader.
I suspect that you may have some inefficient script solutions. For example if you have a # Page of # Pages object on the Master Page, then both of these will have script in the layout:ready event.
The layout:ready event fires EVERY TIME the form changes. So if the user inputs data into a field, 200 layout:ready events would fire: # Page of # Pages object on 100 pages.
Have a look at a discussion here: http://assure.ly/nB0Bvz.
John Brinkman has a few degugging tools on his blog that are well worth checking out: http://blogs.adobe.com/formfeed/. Also check out his general advice here: http://blogs.adobe.com/formfeed/2010/02/big_and_complex_forms.html.
Hope that helps,
Niall