How to debug whats happening when it looks like nothing is happening

Hi everyone, so I got a strange one here

How do I debug a webpage that looks fully loaded but actually isn’t, when all the network activity has stopped, and everything seems static, but the info that should be displayed is not, and then about 45 seconds later it suddenly appears.

If you feel like watching a very boring 1 minute video with no sound of my mouse bouncing around and a timer running, here it is. Im not overselling it when i say this video is boring.