Thermal throttling (CPU@400MHz) running in Coolermaster case

this is more to help other people from wasting time tracking this one down. I have a 13th Gen Intel 1370p (with 64GB RAM/ WD 850x/wifi) which I’ve run in a Cooler master case since new (I also have a proper Framework 13 laptop running Intel 1340p with 64GB RAM and WD850x too).
Recently it started to show dreadful performance - it is running Windows 11 23H2 and the obvious thing is that there was a software update which didn’t like the motherboard, so I tried all the obvious things of backing out updates, looking at the BIOS etc and even resorting to things like ThrottleStop and ParkControl, but nothing would make the machine run consistently well. My laptop performs beautifully and was wiping the floor with the ‘larger’ desktop running in the case.
ResourceMonitor showed that the CPU was running at a maximum of 20% CPU (so around 400MHz for a 1370p) and with the Intel Extreme Tuning tool running, it showed the processor was running at around 60C whilst consuming only 7W of power which is completely wrong.
I opened the case up and found a significant amount of fluff around the cooling fan, and with it only being a laptop fan, the blades are small and close together so fluff has a massive effect on it. The fluff was duly removed and all cooling components blasted with compressed air.
I put it back together again, fired it up and it is now perfect again - no throttling, no slow performance and even allowing for Windows 11 23H2, it is quite responsive once more.
So if anyone else experiences the CPU being throttled back to 400MHz or terrible performance, check the fans - they may be full of stuff and a simple clean gets it all going again.
I hope this helps someone else and saves them the hair loss and time I did trying to sort this out.