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/fields.py
""" The value of such a field is a recordset of size 0 (no
    record) or 1 (a single record).
    :param comodel_name: name of the target model (string)
    :param domain: an optional domain to set on candidate values on the
        client side (domain or string)
    :param context: an optional context to use on the client side when
        handling that field (dictionary)
    :param ondelete: what to do when the referred record is deleted;
        possible values are: ``'set null'``, ``'restrict'``, ``'cascade'``
    :param auto_join: whether JOINs are generated upon search through that
        field (boolean, by default ``False``)
    :param delegate: set it to ``True`` to make fields of the target model
        accessible from the current model (corresponds to ``_inherits``)
    The attribute ``comodel_name`` is mandatory except in the case of related
    fields or field extensions.
    """
    type = 'many2one'
    column_type = ('int4', 'int4')
    _slots = {
        'ondelete': 'set null',         
        'auto_join': False,             
        'delegate': False,              
    }
    def __init__(self, comodel_name=Default, string=Default, **kwargs):
        super(Many2one, self).__init__(comodel_name=comodel_name, string=string, **kwargs)
    def _setup_attrs(self, model, name):