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
""" with_prefetch([prefetch]) -> records
        Return a new version of this recordset that uses the given prefetch
        object, or a new prefetch object if not given.
        """
        return self._browse(self._ids, self.env, prefetch)
    def _convert_to_cache(self, values, update=False, validate=True):
        """ Convert the ``values`` dictionary into cached values.
            :param update: whether the conversion is made for updating ``self``;
                this is necessary for interpreting the commands of *2many fields
            :param validate: whether values must be checked
        """
        fields = self._fields
        target = self if update else self.browse([], self._prefetch)
        return {
            name: fields[name].convert_to_cache(value, target, validate=validate)
            for name, value in values.iteritems()
            if name in fields
        }
    def _convert_to_record(self, values):
        """ Convert the ``values`` dictionary from the cache format to the
        record format.
        """
        return {
            name: self._fields[name].convert_to_record(value, self)
            for name, value in values.iteritems()
        }
    def _convert_to_write(self, values):