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
```
avoid_models = {
            parent_model
            for parent_model, parent_field in self._inherits.iteritems()
            if parent_field in values
        }
        
        missing_defaults = {
            name
            for name, field in self._fields.iteritems()
            if name not in values
            if name not in MAGIC_COLUMNS
            if not (field.inherited and field.related_field.model_name in avoid_models)
        }
        if not missing_defaults:
            return values
        
        defaults = self.default_get(list(missing_defaults))
        for name, value in defaults.iteritems():
            if self._fields[name].type == 'many2many' and value and isinstance(value[0], (int, long)):
                
                defaults[name] = [(6, 0, value)]
            elif self._fields[name].type == 'one2many' and value and isinstance(value[0], dict):
                
                defaults[name] = [(0, 0, x) for x in value]
        defaults.update(values)
        return defaults
    @classmethod
    def clear_caches(cls):