On large recordsets I got some severe performance issues while browsing through an inherits relation.
For example I have a batch process exporting a csv file with products variants:

I have to export 50.000 products.

If I do :
`variant.categ_id` It takes a very long time to get the result.

If I do:
`variant.product_tmpl_id.categ_id` The result is immediate

The problem is that the global perfomance of the application is impacted by this problem.

For example my POS takes more than 15 minutes to load. Because I have 26.000 products and the compute price method uses a lot browsing through the inherits such like `product.cated_id`, `product.uom_id` and they all takes a huge time to compute.

Same issue in this case.

If I do :
`variant.price` It takes a very long time to get the result.

If I do:
`variant.product_tmpl_id.price` The result is immediate

Could you help me ?


Simple way to reproduce:

1/ Create a new database and install sale module
2/ Launch following script through odoo shell :

```python
# -*- coding: utf-8 -*-
import logging

_logger = logging.getLogger(__name__)
self = self  # noqa

nb_product = 50000

i = 0
while i < nb_product:
    prod = self.env['product.template'].create({'name': 'product_%s' % i})
    i += 1
    if i % 1000 == 0:
        self.env.cr.commit()
        _logger.info("%s Products Created" % i)
```

2/ Launch following script through odoo shell :
```python
# -*- coding: utf-8 -*-
import logging

_logger = logging.getLogger(__name__)
self = self  # noqa


products = self.env['product.product'].search([])

for product in products:
    # Immediate response
    _logger.info('Category Template %s' % product.product_tmpl_id.categ_id)
    # Lagging response
    _logger.info('Category Variant %s' % product.categ_id) 
    break
```
Sparse fields have a very small probability of being not null. Therefore
        many such fields can be serialized compactly into a common location, the
        latter being a so-called "serialized" field.
        :param sparse: the name of the field where the value of this field must
            be stored.
        .. _field-incremental-definition:
        .. rubric:: Incremental definition
        A field is defined as class attribute on a model class. If the model
        is extended (see :class:`~odoo.models.Model`), one can also extend
        the field definition by redefining a field with the same name and same
        type on the subclass. In that case, the attributes of the field are
        taken from the parent class and overridden by the ones given in
        subclasses.
        For instance, the second class below only adds a tooltip on the field
        ``state``::
            class First(models.Model):
                _name = 'foo'
                state = fields.Selection([...], required=True)
            class Second(models.Model):
                _inherit = 'foo'
                state = fields.Selection(help="Blah blah blah")
    """
    __metaclass__ = MetaField
    type = None                         
    relational = False                  
    translate = False                   
    column_type = None                  
    column_format = '%s'                
    _slots = {
        'args': EMPTY_DICT,