May I ask why Nextcloud's handle video streaming so well? (It buffers in advance very well)

Hi,

I am currently using nextcloud:18.0.12-fpm-alpine docker image to self-host a cloud.
I recently found Nextcloud’s handle video streaming very well (i.e. I found it preloaded things very smart).

Does anyone know what trick did Nextcloud use in the code?

Because I want to enhance an open source social network kit called Humhub, where I found its video streaming was painfully slow.

Can somebody point out how Nextcloud handle video streaming so well?

As you can see, it buffered in advance. I just wondered where is the code that tells it to do so.