Perhaps it can be done here without calling the method in a loop

[salitygen]

joomla-cms/plugins/system/fields/fields.php

Line 165 in de15a40

$model->setFieldValue($field->id, $item->id, $value);

I think it is possible to optimize here by passing values once to method setFieldValue in the form of an array or object

After that, of course, you will need to rewrite this method

joomla-cms/administrator/components/com_fields/src/Model/FieldModel.php

Line 611 in de15a40

public function setFieldValue($fieldId, $itemId, $value)

And send fields to the database in groups, not one at a time. This will significantly speed up the process of group operations for materials with a large number of custom fields