Using `read_group` to gather such a sum is always more performant
than a `search` followed by a `mapped`.

In the case of the studied database,
the computation never ended,
while it ends in a few seconds
using `read_group`.

upg-7528

closes odoo/odoo#66502

X-original-commit: 8547bb6145c3ddf9c7205ea10deef22473362cce
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>