For myself, I’d not use htop but instead vmstat 5 because that leaves a clear text log of what it saw (every 5 seconds). That’s handy because you can scroll back, or search for the worst moment. The worst moment is what matters, really, especially if it causes failure. The average situation (during a rebuild) isn’t informative. With a tool like htop you could easily miss seeing the peak, and once it’s past you can’t review.
I’ve posted before about vmstat and explained how to interpret the output.