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
""" Apply ``func`` on all records in ``self``, and return the result as a
            list or a recordset (if ``func`` return recordsets). In the latter
            case, the order of the returned recordset is arbitrary.
            :param func: a function or a dot-separated sequence of field names
                (string); any falsy value simply returns the recordset ``self``
        """
        if not func:
            return self                 
        if isinstance(func, basestring):
            recs = self
            for name in func.split('.'):
                recs = recs._mapped_func(operator.itemgetter(name))
            return recs
        else:
            return self._mapped_func(func)
    def _mapped_cache(self, name_seq):
        """ Same as `~.mapped`, but ``name_seq`` is a dot-separated sequence of
            field names, and only cached values are used.
        """
        recs = self
        for name in name_seq.split('.'):
            field = recs._fields[name]
            null = field.convert_to_cache(False, self, validate=False)
            recs = recs.mapped(lambda rec: field.convert_to_record(rec._cache.get(field, null), rec))
        return recs
    def filtered(self, func):
        """ Select the records in ``self`` such that ``func(rec)`` is true, and
            return them as a recordset.
            :param func: a function or a dot-separated sequence of field names