I've noticed a curious trend with the WUs for 5900X system recently, they seem to have intermittent periods where disk usage goes up significantly and CPU usage goes down.

This has meant tasks now run for far longer than they used to, tasks used to take about 8000 - 11000 seconds, and almost all of it was CPU time.

Now tasks take anywhere from 15000 - 30000 seconds, with the same ~8-11k seconds of CPU time.

Can anyone shed light on what's going on here?

Are you running any other CPU intensive software? If you are try closing it. I would also try restarting the computer. Feel free to have a look at my system details in my account I am running the same CPU as you are and my times are under 3 hours