Hi,

You can add the field to the database table using SQL query, which will skip the computation for the existing records. 

See an optimization proposed here: https://github.com/odoo/odoo/pull/120328/files

Sample from odoo source code:

class AccountMoveLine(models.Model):
    _inherit = "account.move.line"

    l10n_pe_group_id = fields.Many2one("account.group", related="account_id.group_id", store=True)

    def _auto_init(self):
        """
        Create column to stop ORM from computing it himself (too slow)
        """
        if not column_exists(self.env.cr, self._table, 'l10n_pe_group_id'):
            create_column(self.env.cr, self._table, 'l10n_pe_group_id', 'int4')
            self.env.cr.execute("""
                UPDATE account_move_line line
                SET l10n_pe_group_id = account.group_id
                FROM account_account account
                WHERE account.id = line.account_id
            """)
        return super()._auto_init()

Or you can initially add the field as a non computed field in the module and install in the db and then make it as a computed field, this will also do the trick.

Thanks