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
return self.id
    def __str__(self):
        return "%s%s" % (self._name, getattr(self, '_ids', ""))
    def __unicode__(self):
        return unicode(str(self))
    __repr__ = __str__
    def __hash__(self):
        if hasattr(self, '_ids'):
            return hash((self._name, frozenset(self._ids)))
        else:
            return hash(self._name)
    def __getitem__(self, key):
        """ If ``key`` is an integer or a slice, return the corresponding record
            selection as an instance (attached to ``self.env``).
            Otherwise read the field ``key`` of the first record in ``self``.
            Examples::
                inst = model.search(dom)    # inst is a recordset
                r4 = inst[3]                # fourth record in inst
                rs = inst[10:20]            # subset of inst
                nm = rs['name']             # name of first record in inst
        """
        if isinstance(key, basestring):
            
            return self._fields[key].__get__(self, type(self))
        elif isinstance(key, slice):
            return self._browse(self._ids[key], self.env)
        else:
            return self._browse((self._ids[key],), self.env)
    def __setitem__(self, key, value):