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
""" Remove the cached value of ``field`` for all ``records``. """
        if isinstance(field, basestring):
            field = self._recs._fields[field]
        field_cache = self._recs.env.cache[field]
        for id in self._recs._ids:
            field_cache.pop(id, None)
    def __iter__(self):
        """ Iterate over the field names with a regular value in cache. """
        cache, id = self._recs.env.cache, self._recs.id
        dummy = SpecialValue(None)
        for name, field in self._recs._fields.iteritems():
            if name != 'id' and not isinstance(cache[field].get(id, dummy), SpecialValue):
                yield name
    def __len__(self):
        """ Return the number of fields with a regular value in cache. """
        return sum(1 for name in self)
AbstractModel = BaseModel
class Model(AbstractModel):
    """ Main super-class for regular database-persisted Odoo models.
    Odoo models are created by inheriting from this class::
        class user(Model):
            ...
    The system will later instantiate the class once per database (on
    which the class' module is installed).
    """
    _auto = True                
    _register = False           
    _abstract = False           
    _transient = False          
class TransientModel(Model):