I have this too - but always just assumed it’s something else on my system. Or someone on the network starting something big. I imagine it would be quite an undertaking to track it down so just left it. But, if you set up logging across those things - HDD’s and firewall as a start and look for correlation of events - that’s how I’d do it.
Look at the log directly as when the slowdown happens it will print the path to the affected plots. Now not all the plots affected maybe contributing to the slowdown but it shows you all the ones affected during the specific challenge. This may give you a clue especially if these plots are on specific HDD.
Unknown reason, today seems back to 1.x seconds maximum… without touching anything …
The sole reasons I can think of … as someone mentioned before is the grid itstelf and that my ISP ain’t as honest as it should be … this influx of time started when more people started doing homework… yet I pay for fiber so shouldn’t have any influence compared to cable unless the bottleneck is fiber → cable → fiber somewhere from my ISP… if that makes sense …