mediasoup-demo_v3 video quality not good

mediasoup demo v3, lan environment:
client_1:, chrome, send 1280x720 vp8(or vp9), svc.
client_2:, chrome, watch client_1 video stream.

At the beginning client_2 can recieve 1280x720 stream from client_1, after 5-15s later, stream always downgrade to 960x540 or 640x360.
Why svc downgrade in such a good lan bandwidth environment? ? Feel no sense^^

Open chrome://webrtc-internals/ if you are using something Chromium-based and it will tell you why exactly quality was downgraded.