CPU: 11th Gen i5
The short title belies a lengthy diagnostic effort. Hopefully this helps someone else. I’m not sure if I’ve got to the end of the issue, but the immediate problem’s gone away.
Symptoms were a general sense of underperformance that didn’t seem to be backed up by the system stats, and particularly bad performance around videos, e.g. on YouTube, but also some local applications - even LibreCalc was stuttering. This persisted across different Linux distros and Windows.
After exploring several tenuously connected issues on the forums, I found this, and verified that there was actually a serious CPU throttling problem (which isn’t obvious from using top and other tools). I’d checked and cleaned inside the case, and maybe the processor could do with more thermal paste, but removing my DP module immediately fixed the visible performance issues. I checked that it didn’t matter which port the module was in, it always brought back the performance issue.
So that’s a huge improvement in what I can do with the laptop, but I’m wondering if it makes sense to check anything else, or I can just blame a faulty DP module. Has anyone else had this problem, and anyone raised a support ticket for it? Any testing suggestions?