Storing huge data in a Viewstate is not recommended. The reasons are;
a) your page becomes very heavy and will take a lot of bandwidth and time to load.
b) ViewState involves extra overload of serializing and deserializing the objects, so involves extra processing time.
c) If not used properly, ViewState will cause performance issues in your page.
So what are your options? You will have to use other alternatives to store the data. Here are some links which will help you understand and use ViewState wisely:
ASP.NET View State Overview[
^] (You may directly scroll down to the section - Considerations for Using View State)
ASP.NET State Management Recommendations[
^]
ASP.NET 2.0 Performance Guidelines - View State[
^]
Hope this helps!