I have gigabit, and struggled to stream. Turned out I had the Quality of Service (QoS) /traffic priority settings on my router misconfigured.

This might be something to look into.

Can a remote user download something from your network at a reasonable and consistent speed? (Not using Plex)

Can you upload a large file somewhere and monitor the speed and see if it maintains speed as expected?

For me, these two things were also performing at unexpectedly low speeds, or being wildly inconsistent until I fixed my QoS settings.

I always thought it was a bad idea for people to treat Discord as a free CDN.

I hope they hit on something stand-out soon. To establish more sustainability. Seems like everything is in change right now.