Welcome to the forum Maureen.
400 items are a lot to load into your form and it takes quite a while. For performance reasons and often usability you want to break this up into smaller lists and then query for the actual items.
So create groupings by type, category, etc and then users can find the item they need in a more organized manner and it will perform much better.
I would suggest cascading dropdowns to narrow your choices or some other type of lookup method.
Email me if you would like to see some examples.