Sounds like a problem that'd hit anyone running LLMs. I haven't tried on mine so far, but people do talk about ordering a $10k Mac Studio just for that. Anyone else see this? Does the OS version matter?

Never.

I don't think I had the issue before macOS 26, but honestly I have no way to tell. I did run into one other HN commenter that suffers from the same slowness problem, but they didn't connect it to workloads, only uptime. I think it was the same issue though.