How to customize tree view loading time?

I am working with the TreeView in OpenERP. But my tree view takes a long time to load. How can customize that?

I have defined a Many2many field in the product.product model.

product_ids = fields.Many2many(
    comodel_name='product.product', 
    relation='product_procurement_rel',
    column1='process_id',
    column2='product_id',
    string='Products'
)

How can I solve this?