How to design scalable video streaming architecture using GCP

Question

I have a program for streaming videos that pulls content from a Google Storage Bucket. All of the files that are stored in the storage bucket are private. I create a signed URL using API every time a user clicks on a video from the front end and load it into the HTML5 video player.

Problem

I observe that it takes approximately 30–40 seconds for the video to load on the front end if the file size exceeds 100 MB.

When I searched online for solutions to this issue, several of the blogs advised using a cloud CDN and a storage bucket before caching the file. As far as I'm aware, the file must be publicly accessible in order to be cached. I am not allowed to publish files.
Therefore, my issue is whether there are any means by which we can make it scalable or shorten the setup period.