As expected, one of my slower processes has rediscovered (this afternoon) Gabor Levai's 1000027-digit Leyland prime. I thought that it might be a good time to see how far I've come. It seems that I've completed about 49% of the numbers that I wanted to test since I started 132 days ago, so, on average, I'm testing about 220 numbers per day. It means that, at best, I'll be done by February 2023 but a lot depends on my transferring processes from my three slow computers to others as they become available.
I had installed differing operating systems on those machines to see what would happen. I saw no improvement. So I'm thinking now that the slow speed arises from the fact that these three Mac minis were purchased (in order to save money) with only 8 GB RAM, which may be insufficient for the task at hand (it had been adequate for testing 300000-digit numbers). I will now terminate three of the six processes on each machine to see if there is any speed improvement on the remaining three processes. Later this month I will have completed 14 processes on two other machines, which allows me to transfer the 9 terminated processes.