Microsoft remote desktop beta2(macOS client) laggy when streaming video with retina optimization

I'm streaming from a 4k windows VM to 2021 macbookPro(around 4k resolution). When enable "optimize retina display" option, it looks very nice but becomes laggy when streaming fullscreen video.

My network is not the bottleneck. I have a wifi7 router with max speed 500Mbps in practice. RDP only uses 30Mbps at most from my observation.

I'm wondering what is the bottleneck that causes the laggy behavior.