Large Table Causing Slow Opening Form

I have a database with a table that contains 360,000 rows. I built a form with four boxes where a user can specify values to limit the result set. And instead of having a new window open with the results, I built a subform and placed it on the main form to display the results.

Here is how it flows:
Main Form -> user enters search criteria
Search Criteria -> feed as criteria in query
Query Results -> display on subform
Subform -> shows on main form

The query is setup to take the values from the main form and either use it if it's not null, or return all values if the field is null.

The problem I am having is that on opening the main form, Access is taking the four null values from the main form search fields, feeding them to the query, which is then feeding the subform. So 10 minutes later when the main form finally opens, I have 360K records displaying in my subform.

What I would like is to be able to open the main form instantly, specify my search criteria, then run the query, then have the query results populate the subform.

What do I need to do?