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
```
""" Export fields for selected objects
            :param fields_to_export: list of fields
            :param raw_data: True to return value in native Python type
            :rtype: dictionary with a *datas* matrix
            This method is used when exporting data via client menu
        """
        fields_to_export = map(fix_import_export_id_paths, fields_to_export)
        if raw_data:
            self = self.with_context(export_raw_data=True)
        return {'datas': self._export_rows(fields_to_export)}
    @api.model
    def load(self, fields, data):
        """
        Attempts to load the data matrix, and returns a list of ids (or
        ``False`` if there was an error and no id could be generated) and a
        list of messages.
        The ids are those of the records created and saved (in database), in
        the same order they were extracted from the file. They can be passed
        directly to :meth:`~read`
        :param fields: list of fields to import, at the same index as the corresponding data
        :type fields: list(str)
        :param data: row-major matrix of data to import