On my Linux system with Brave browser and server grade CPUs (96 cores), RAM (512 GB) and a Nvidia 4060 the steam causes the entire browser to become very slowly but steadily increasingly sluggish until the browser is manually restarted. At first only the video is lagging (audio is fine), then a few hours later it affects the entire site, and a few more hours later its the entire browser. DISABLING the Widevine plugin and hardware acceleration within the browser fixes this issue.

Downgrading the resolution temporarily works around the issue, but a few hours later the issue is again as bad but at the lowered resolution.

So far I've only noticed it on Twitch.
Amazon Prime (wich is broken WITHOUT the Widevine plugin) did NOT show such issue at all.

Therefore I assume your assistance is needed to further nail down this bug. And get it addressed at the right place (either you, as it only appears to happen on your side), at Brave, or by Widevine.

Therefore, is there anything special you're doing when Widevine and software acceleration are present that someone else may not be doing? Special Codecs or something? Is there any knowledge that you could provide in this regards that could help here?

(Furthermore no adblocker or similar are enabled and the builtin one in Brave has been disabled for the twitch site).

When it is working the developer console on your site logs "[VideoTransformer] WebGPU adapter unavailable", so could this be a bug in regards to that feature? Sadly I don't know anything about this.