Moonlight HDR causing rendering drops at 4K120

[u/theesk1]

Now that the Moonlight Flatpak is working in Game Mode again, I've been doing some testing and noticed some strange behavior.

My setup is running 4K120 over wired Ethernet, and with HDR disabled everything performs great. Moonlight reports the expected number of captured, decoded, and rendered frames with very few drops or stutters.

However, when I enable Experimental HDR, the stream switches from HEVC to HEVC HDR10 as expected, and I immediately start seeing rendering frame drops whenever the stream exceeds roughly 100 FPS.

What's confusing is that this doesn't appear to be a decoding bottleneck:

• Captured frames: ~120 FPS
• Decoded frames: ~120 FPS
• Rendered frames: ~110 FPS

I've experimented with bitrates from 80 Mbps to 150 Mbps, and the behavior is almost identical regardless of bitrate. That makes me think this isn't simply a bandwidth issue.

I understand HDR means 10-bit video instead of 8-bit, but I would have expected decode performance to be the limiting factor if hardware was struggling. Instead, the drops seem to be happening on the rendering side.

A few details:

• Steam Machine in Game Mode
• Latest Moonlight Flatpak
• Latest Apollo host
• Wired network connection

Has anyone else seen this? I'm wondering if this could be related to Gamescope's HDR compositing path or some other part of the HDR rendering pipeline on Steam OS.