Upload speed takes 10 minutes to reach 8Mbps

I have been asked by a friend the following question; Why does it take 10 minutes for the upload speed to reach 8Mbps when streaming live church service to YouTube?
BT Package Your normal available download speed will be between 49-50 Mbps
Your normal available upload speed will be between 8-9 Mbps
The pc used to stream is connected via Ethernet cable from BT Hub located on an adjacent building connected via catenary wire.
On a normal day without streaming the upload speed is usually 8.34Mbps.
When the stream is first initially started the upload speed is running at 1 or 2Mbps and it takes 10 minutes to reach 8Mbps.
What would be the reason for this to happen?

My best guess would be that the YouTube streaming system gradually ramps up because it doesn't know how much upstream bandwidth it really has available.

The same sort of thing happens on the downstream side when streaming from places like YouTube, Netflix etc They'll start on a low rate stream and gradually move to higher bit rate streams as they get more confident that enough bandwidth is available.

This is possibly why their live streaming tips include:

"Start encoder(s) at least 15 minutes before event is scheduled to start."

https://support.google.com/youtube/answer/2853856?hl...

Thanks @andynormancx. That makes sense. I shall pass it on to my friend.