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
```
""" Convert ``value`` from the record format to the format returned by
        method :meth:`BaseModel.onchange`.
        :param fnames: an optional collection of field names to convert
            (for relational fields only)
        """
        return self.convert_to_read(value, record)
    def convert_to_export(self, value, record):
        """ Convert ``value`` from the record format to the export format. """
        if not value:
            return ''
        return value if record._context.get('export_raw_data') else ustr(value)
    def convert_to_display_name(self, value, record):
        """ Convert ``value`` from the record format to a suitable display name. """
        return ustr(value)
    
    #
    
    #
    def check_schema(self, model):
        """ Make sure the database contains everything for this field. Return
        ``True`` if the schema was altered and the field must be recomputed.
        """
        pass
    def read(self, records):
        """ Read the value of ``self`` on ``records``, and store it in cache. """
        return NotImplementedError("Method read() undefined on %s" % self)
    def write(self, records, value):