Slow load from External Folder

I downloaded some videos ( approx. 200videos, total 30G) and transferred them to my iPhone and keep the folder available for other applications visiting.

After I added the sources from external folders, it is very slow to establish local index. 5-30s.

Digit code for your reference: QJ1SF, 3R23A

Question is, why it requires a long time to load local videos…