Session Vars and Performance

I have a multi-page form which I build up and store in session variables. The data saved includes all an internal list of items on the form (derived from a database table), all the form field specs (derived from the internal item list), the data for the fields (from another table), default data for restting the form (from yet another table), and a data structure for my forms class. It adds up to about 250KB in the actual session file -- that is, the serialized data. I'm not clear of the impact this will have in performance when it's in a multi-user environment, and I'm not sure at what point the overhead of serializing and unserializing gets to be more than the overhead of sticking this stuff in a temporary database table and then retrieving it. Serializing is simpler but my git says there has to be a point at which it gets inefficient. Testing is complex since I would have to write all the database code in order to do any performance measurement. Anyone have relevant experience in this area, and/or some educated guesses? Thanks, -- Tom