I hope this helps you . Magento has an ability to trigger the index event on every product update action. In your case you are trying to update product by bulk action. Hence, magento triggers the full reindexing.
Magento API: Rebuild Indexes after adding new products
By : haio
Date : March 29 2020, 07:55 AM
hope this fix your issue I am currently writing a script that lets me import multiple products in magento. , You can use such a model in Index module.