Check your RAM usage. If your system is out of RAM during the simulation it will use your hard-drive as RAM (paging memory). This is much slower if you don't have a SSD hard-drive. SSD is a bit slower than RAM, but acceptable. On my system the CPU is eventually the bottleneck if SSD is used as RAM.
Thanks. I have mostly solved the issue. I have a piece of complex geometry (a loft/blend) that I replaced with something simpler, and iteration time went from 30-40 minutes to 10 minutes, which is more in the expected range given the increased number of cells vs previous runs.
Any further advice on bringing this down would be helpful as well.