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
```
odoo/models.py
""" fields_get([fields][, attributes])
        Return the definition of each field.
        The returned value is a dictionary (indiced by field name) of
        dictionaries. The _inherits'd fields are included. The string, help,
        and selection (if present) attributes are translated.
        :param allfields: list of fields to document, all if empty or not provided
        :param attributes: list of description attributes to return for each field, all if empty or not provided
        """
        has_access = functools.partial(self.check_access_rights, raise_exception=False)
        readonly = not (has_access('write') or has_access('create'))
        res = {}
        for fname, field in self._fields.iteritems():
            if allfields and fname not in allfields:
                continue
            if field.groups and not self.user_has_groups(field.groups):
                continue
            description = field.get_description(self.env)
            if readonly:
                description['readonly'] = True
                description['states'] = {}
            if attributes:
                description = {key: val
                               for key, val in description.iteritems()
                               if key in attributes}
            res[fname] = description
        return res
    @api.model
    def get_empty_list_help(self, help):