The method `read()` may be very slow when reading relational fields and
computed fields, because the computed fields can be computed on a recordset
that is larger than expected.
The issue occurs on model 'res.partner' when reading fields 'child_ids' and
'purchase_order_count', for instance. Suppose we read those two fields on a
partner with 1000 contacts. First, the one2many field is read from the
database and stored to the cache; the latter adds the value ids to the
prefetching of 'res.partner'. Then, the fields are fetched from the cache.
When 'purchase_order_count' is accessed on the partner, the field is computed
on all its children as well...
closesodoo/odoo#29867
Raphael Collet