Oracle 10g slow performance on first query
The usual reason for such issues is that the first execution of the query requires physical reads to fetch the required blocks from disk, and the second and subsequent executions find that they are already cached.

You can test this by running the queries from SQL*Plus (and timing them) prior to first opening these windows of which you speak, to see if that affects performance.

If you can come back with comparison of the timings when running the queries through SQL*Plus and then through the application it would help further analysis.