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
removes the record of id ``id`` from the set, but does not
              delete it. Can not be used on
              :class:`~odoo.fields.One2many`. Can not be used in
              :meth:`~.create`.
          ``(4, id, _)``
              adds an existing record of id ``id`` to the set. Can not be
              used on :class:`~odoo.fields.One2many`.
          ``(5, _, _)``
              removes all records from the set, equivalent to using the
              command ``3`` on every record explicitly. Can not be used on
              :class:`~odoo.fields.One2many`. Can not be used in
              :meth:`~.create`.
          ``(6, _, ids)``
              replaces all existing records in the set by the ``ids`` list,
              equivalent to using the command ``5`` followed by a command
              ``4`` for each ``id`` in ``ids``.
          .. note:: Values marked as ``_`` in the list above are ignored and
                    can be anything, generally ``0`` or ``False``.
        """
        if not self:
            return True
        self._check_concurrency()
        self.check_access_rights('write')
        
        for field in itertools.chain(MAGIC_COLUMNS, ('parent_left', 'parent_right')):
            vals.pop(field, None)
        
        old_vals, new_vals, unknown = {}, {}, []
        for key, val in vals.iteritems():
            field = self._fields.get(key)