The way you drop latency is by reducing or eliminating buffers, that's pretty much it.
There's a trade-off with h264 and hevc/265, as with most video codecs - the larger the buffer you have, the more delta frames you can have between key frames.
This means lower bandwidth and better compression.
So it's a constant struggle between compression and latency.
The lowest latency that webrtc can get is about 4ms one way, 8ms round trip, at least based on the project I started to do realtime steaming with https://github.com/3DStreamingToolkit/3DStreamingToolkit
We proved you could do round trip times in the real world from cloud to customer in under 25ms.
But it comes with a lot of tradeoffs and cost and complexity to make it work.
Google Stadia fundamentally uses webrtc under the covers, although they use QUIC instead of TCP/ICE.
If you ripped out all of the congestion management from the video engine in webrtc, you'd reduce another several ms roundtrip at the cost of basically no network resilience.
Edit: the topic of my bachelor's thesis included low latency live streaming, so I hope I have at least some knowledge :)
Getting buffers down to 500ms is already pretty insane assuming reliability doesn't suffer. 250ms is well within the envelope of a single drop on a 3G network killing playback